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STATE STREET INSTITUTIONAL INVESTMENT TRUST

 ROPES & GRAY LLP

 PRUDENTIAL TOWER

800 BOYLSTON STREET

 BOSTON, MA 02199-3600

WWW.ROPESGRAY.COM

 Adam M. Schlichtmann

 617-951-7114

617-235-7346 fax

 adam.schlichtmann@ropesgray.com

 December 16, 2021

VIA EDGAR CORRESPONDENCE

 Securities and Exchange
Commission

 Division of Investment Management

 100 F Street,
N.E. Washington, D.C. 20549

 Re: State Street Institutional Investment Trust (the “Trust”) (SEC File No. 333-30810 and 811-09819) Amendment
Number 285 (the “Amendment”) to the Trust’s Registration Statement (the “Registration Statement”) on Form N-1A Filed on October 8, 2021, Pursuant to Rule 485(a) under the Securities Act of 1933, as amended (the
“1933 Act”)

 Dear Mr. Esperon

 This letter
provides the Trust’s responses to comments on the Amendment relating to the registration of the State Street Diversified Income Fund (the “Fund”), a new series of the Trust, under the Investment Company Act of 1940, as amended (the
“1940 Act”) and the registration for offering of certain shares of the Fund under the 1933 Act that Mr. Jeremy Esperon of the staff (the “Staff”) of the Securities and Exchange Commission (the “SEC”) provided by
telephone to Adam Schlichtmann on November 19, 2021. For convenience of reference, each of the comments is summarized before the Trust’s responses. Capitalized terms not otherwise defined herein have the meanings ascribed to them in the
Registration Statement.

 General

1.
 Comment: Where a comment is made in one location, it is applicable to all similar disclosure appearing
elsewhere in this Registration Statement. We remind you that the Fund and management are responsible for the adequacy and accuracy of all disclosure notwithstanding any review, action, or absence of action by the Staff.

 Response: The Fund acknowledges this comment.

2.
 Comment: Please file the response at least 5 business days in advance of the effective date.

 Response: The Fund acknowledges this comment.

3.
 Comment: Please confirm that tickers will be added on EDGAR when available.

 Response: The Fund confirms that tickers will be added on EDGAR when available.

 Prospectus

4.
 Comment: Please bold the language that reads “You may pay other fees, such as brokerage commissions
and other fees to financial intermediaries which are not reflected in the tables and examples below” in the section titled “Fees and Expenses of the Fund.”

 Response: In response to this comment, the Fund will bold the above referenced language.

5.
 Comment: Please include the final fee and expense table and expense examples in your response to our
comments.

 Response: The Fund’s final fee and expense tables will appear as follows:

Fees and Expenses of the Fund

 The tables below describe
the fees and expenses that you may pay if you buy, hold, and sell shares of the Fund (“Fund Shares”). You may pay other fees, such as brokerage commissions and other fees to financial intermediaries which are not reflected in the
tables and examples below.

 Shareholder Fees (fees paid directly from your investment)

Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price)

None

Maximum Deferred Sales Charge (Load) (as a percentage of the lower of the sale proceeds or the original offering price)

None

 Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your
investment)

 Management Fee

0.25
%

 Distribution and/or Shareholder Service (12b-1) Fees

0.00
%

 Other Expenses1

0.83
%

 Total Annual Fund Operating Expenses

1.08
%

 Less Fee Waivers and/or Expense
Reimbursements2

(0.38
)%

 Total Annual Fund Operating Expenses After Fee Waivers and/or Expense Reimbursements

0.70
%

1
 Other Expenses are based on estimates for the current fiscal year.

2
 The Fund’s investment adviser, SSGA Funds Management, Inc. (the “Adviser” or “SSGA
FM”), is contractually obligated until April 30, 2023 (i) to waive up to the full amount of the advisory fee payable by the Fund, and/or (ii) to reimburse the Fund to the extent that Total Annual Fund Operating Expenses
(exclusive of non-recurring account fees, interest, taxes, and extraordinary expenses) exceed 0.70% of average daily net assets on an annual basis. This waiver and/or reimbursement may not be terminated prior to April 30, 2023 except with
approval of the Fund’s Board of Trustees.

 Example:

This Example is intended to help you compare the cost of investing in the Fund with the cost of investing in other funds. The Example assumes that you invest
$10,000 in the Fund for the time periods indicated, and then sell or hold all of your Fund Shares at the end of those periods. The Example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain
the same. The calculation of costs for the one-year period takes into account the effect of any current contractual fee waivers and/or reimbursements; and the calculation of costs for the remaining period takes such fee waivers and/or reimbursements
into account only for the first year of each such period. Although your actual costs may be higher or lower, based on these assumptions your costs would be:

 -2-

 1 year

 3 years

 $72

$306

6.
 Comment: Reference is made to the disclosure in the Section titled “Principal Investment
Strategies” that states “SSGA Funds Management, Inc. (the “Adviser”) attempts to invest the Fund’s assets across higher income-generating sectors while making adjustments to the portfolio to manage risk through different
credit cycles.”

a.
 Please add disclosure that elaborates on how the “higher income-generating sectors” are identified.
If the Fund expects to invest significantly in any particular sector, please add corresponding strategy and risk disclosure.

Response: The Fund will revise the disclosure as follows in response to this comment and the following two comments (added language in
blue underline; deleted language in red strikethrough):

b.
 The Fund seeks to achieve its investment objective by investing under normal circumstances at least 65% of its
net assets in income securities of varying maturities. SSGA Funds Management, Inc. (the “Adviser”) attempts to
investdiversify the Fund’s assets across higher
income-generating sectorstypes of securities while making adjustments to the portfolio to manage
risk through different credit cycles based on the Adviser’s risk/return forecasts. The Fund seeks to reducemanage volatility through portfolio diversification across sectors
different income-generating securities and
managemanaging exposures to duration, credit rating/quality, geography, maturity and different parts of the
capital structure of issuers in which the Fund invests.

7.
 Comment: Please add disclosure that clarifies the meaning of “adjustments to the portfolio to
manage risk through different credit cycles.”

 Response: Please see the Fund’s response above. The Fund
will clarify this phrase by adding a reference to the Adviser’s “risk/return forecasts.”

8.
 Comment: Reference is made to the disclosure in the Section titled “Principal Investment
Strategies” that states “The Fund seeks to reduce volatility through portfolio diversification across sectors and managing exposures to duration, credit rating/quality, geography, maturity and different parts of the capital structure of
issuers in which the Fund invests.” Reference is also made to the disclosure in the same section that reads “The Fund may invest up to 80% of its net assets in high yield securities…” and “The Fund may invest up to 75% of
its net assets in income securities and instruments of emerging market countries.” Please supplementally explain how investments in high yield securities and investments in emerging markets are consistent with the concept of reducing volatility
referenced in the first sentence.

 -3-

 Response: The Fund will revise its disclosure to change “reduce volatility” to
“manage volatility.” Supplementally, the Adviser intends to build an income-oriented portfolio that includes assets in addition to high yield corporate bonds, so it manages volatility by investing in investment grade bonds that introduce
more interest rate sensitivity and mortgage-backed securities that introduce more cash flow sensitivity to reduce the Fund’s exposure to default risk associated with the high yield corporate bonds.

9.
 Comment: Please specify what criteria the Fund will consider in selecting the duration of the securities
it holds. For example, if the Fund can invest in securities of any duration, please add disclosure stating that. Or, if the Fund has a duration target, please add disclosure stating that.

 Response: The Fund will add disclosure that it can invest in securities of any duration.

10.
 Comment: Reference is made to the disclosure in the Section titled “Principal Investment
Strategies” that states “The Fund’s investments may include, but are not limited to, income securities of any credit rating…” Please delete “but are not limited to.”

 Response: In response to this comment, the Fund will delete “but are not limited to” from the
quoted sentence.

11.
 Comment: Reference is made to the disclosure in the Section titled “Principal Investment
Strategies” that states “The Fund may also invest up to 10% of its net assets in convertible securities…”

a.
 If the Fund will invest in contingent convertible securities (“CoCos”), please consider adding
additional disclosure regarding these instruments. The type and location of the disclosure should depend on the extent and characteristics of the CoCos (e.g., their credit quality and conversion triggers). If CoCos are or will be a principal
investment strategy of the Fund, please provide a description of them and appropriate risk disclosure.

 Response: The Trust confirms that the Fund does not intend to invest in CoCos as part of its principal
investment strategy.

b.
 Please inform the Staff supplementally whether the Fund intends to invest in CoCos and, if so, the expected
amount.

 Response: Please see the response to Comment 10(a) above. CoCos are expected to constitute less than 1%
of the Fund’s holdings. The Fund intends to get exposure to preferred securities by investing in one or more underlying exchange traded funds that may have exposure to CoCos through the exchange traded fund’s reference index holdings.

 -4-

c.
 Please also inform the Staff supplementally that CoCos will not be counted as equity securities for purposes of
the Fund’s 80% test.

 Response: Please see the response to Comment 10(a).

12.
 Comment: Please include a separate line item for Acquired Fund Fees and Expenses in the Fund Operating
Expenses table if such expenses related to the Fund’s investments in exchange traded funds (“ETFs”) exceed one basis point. Otherwise, please confirm that the line item “Other Expenses” includes such fees and expenses, if
any.

 Response: The Trust confirms that Acquired Fund Fees and Expenses are not expected to exceed one basis point
and that the line item “Other Expenses” will include such fees and expenses, if any.

13.
 Comment: If the Fund intends on investing principally in distressed or defaulted securities, please add
appropriate principal investment strategy and risk disclosure.

 Response: The Fund does not intend on investing
principally in distressed or defaulted securities.

14.
 Comment: Reference is made to the disclosure in the Section titled “Principal Investment
Strategies” that states “The Fund may invest up to 75% of its net assets in income securities and instruments of emerging market countries.” Please consider clarifying to distinguish whether the 75% policy applies to each of income
securities and emerging market countries, or the two combined.

 Response: The Fund will revise the disclosure as
follows in response to this comment and the following comment (added language in blue underline; deleted language in red strikethrough):

“The Fund may invest up to 75% of its net assets in income securities and instruments of
issuers domiciled in emerging market countries.”

15.
 Comment: Reference is made to the disclosure in the Section titled “Principal Investment
Strategies” that states “The Fund may invest, without limitation, in derivative instruments, such as forwards, options, futures and swaps.” Please delete the words “such as.” Please disclose for what purpose the Fund will
use derivatives, consistent with the Letter from Barry D. Miller to Karrie McMillan (July 30, 2010) (the “Barry Miller Letter”).

Response: In response to this comment, the Fund intends to revise the relevant disclosure as follows (added language in blue underline;
deleted language in red strikethrough):

 The Fund may invest, without limitation, in derivative instruments, such aswhich the Fund
currently expects would include fixed income-related forwards, options, futures and swaps. The Fund expects to use such instruments primarily to adjust market exposures (including duration and yield curve), to gain
broad income market exposures pending investments of cash, or to reduce market exposures pending the sales
of securities. The Fund may also enter into foreign currency futures, forward contracts, and options to hedge currency risk.

 -5-

16.
 Comment: Please add Item 4 or Item 9 disclosure regarding how SSGA Funds Management determines
to sell an investment.

 Response: The Fund will add the following statement to its Item 9 Principal
Investment Strategy disclosure in response to this comment:

 “The Fund generally seeks to buy and hold securities but may sell
securities (i) if there is significant deterioration in the investment case for a particular issuer, (ii) to generate liquidity or (iii) for other portfolio management purposes deemed appropriate by the Adviser.”

17.
 Comment: Please add risk disclosure that corresponds to each of the instruments that the Fund intends on
investing in as discussed in the principal investment strategy.

 Response: In response to this comment, the Fund
intends to add additional risk disclosure related to short sales and collateralized loan obligations, as reflected in Appendix A to this letter.

18.
 Comment: Please add Item 4 or Item 9 risk disclosure specific to the derivative instruments
discussed in the principal investment strategy.

 Response: The Fund has reviewed its disclosure and believes it is
appropriate in light of the nature of the Fund’s intended use of derivatives. The Fund does not intend to make any changes to its risk disclosure in response to this comment.

19.
 Comment: The Staff notes that depositary receipts are mentioned in Non-U.S. Securities Risk. Please add
a reference to depositary receipts in the principal investment strategy and add specific risk disclosure regarding depositary receipts.

Response: The Fund will revise its disclosure as follows in response to this comment:

The Fund may invest in such income securities of issuers located in the United States and non-U.S. countries, including emerging market
entities and may obtain exposure to non-U.S. countries directly or indirectly, such as through investment in depositary receipts.

20.
 Comment: The Staff notes that Equity Investing Risk is included as a principal risk. If investment in
equity securities is part of the Fund’s strategy, please add corresponding disclosure to the principal investment strategy. Otherwise, please remove the risk disclosure.

 Response: The Fund will revise its disclosure to remove the Equity Investment Risk.

 -6-

21.
 Comment: The Staff notes that Exchange Traded Products Risk is included as a principal risk. If
investment in exchange traded products (“ETPs”) is part of the Fund’s strategy, please add corresponding disclosure to the principal investment strategy. Please also revise the risk disclosure to distinguish between the risks of ETPs
and ETFs.

 Response: The Fund has reviewed its risk disc