Janus Investment Fund

Janus Adaptive Global Allocation Fund

Supplement dated October 20, 2016

to Currently Effective Prospectuses

On September 15, 2016, the Board of Trustees of Janus Adaptive Global Allocation Fund (the “Fund”) approved changes to the investment strategies and benchmark indices of the Fund. These changes, each of which is discussed in this Supplement, are intended to provide the Fund with more flexibility to invest across global equity investments and global fixed-income investments, and at times, invest in commodity-linked investments, without having to allocate its investments across these asset classes in any fixed proportion. In addition, these changes will limit the Fund’s use of derivatives. These changes are effective on or about January 1, 2017.

The Fund’s benchmark indices will change as follows:

The Fund will change its primary benchmark index from the Adaptive Global Allocation 70/30 Index to the MSCI All Country World Index^SM. A blended benchmark, which is an internally-calculated, hypothetical composition of total returns from the MSCI All Country World Index^SM (60%) and the Bloomberg Barclays Global Aggregate Bond Index (40%) will be added as a secondary benchmark index for the Fund. The Fund will continue to retain the Bloomberg Barclays Global Aggregate Bond Index as an additional secondary benchmark index.

Janus Capital believes that the foregoing change to the Fund’s primary benchmark index better reflects the Fund’s investments across global markets. Janus Capital believes that the addition of the blended benchmark serves as a reference benchmark that reflects the general mix of asset classes, sectors, and regions in which the Fund may invest.

The following replaces in its entirety the “Principal Investment Strategies” found in the Fund Summary section of the Prospectuses for the Fund:

PRINCIPAL INVESTMENT STRATEGIES

The Fund seeks to achieve its investment objective by allocating its assets across a portfolio of global equity, global fixed-income, and, at times, commodities investments. In doing so, the Fund will attempt to reduce the risk of significant loss, or a drop in the value of the Fund’s capital that is unlikely to be regained over a full market cycle (a time period representing a significant market decline and recovery), while also participating in the upside growth of the capital markets. To achieve this objective, the Fund’s portfolio managers employ a “tail managed” strategy intended to tactically shift away from assets whose downside tail risks are perceived to be increasing and toward assets whose expected tail gains are increasing. As it relates to investing, “tails” represent the outliers of a distribution of returns or, in other words, outsized future moves both to the positive and negative. Tail events typically occur more often than expected, and a tail loss or a tail gain can have a substantial impact on a portfolio’s long-term performance.

The Fund uses a variety of investments to gain the desired exposure, including global equities, global fixed-income securities, and with respect to commodity-linked investments, exchange-traded funds (“ETFs”) and mutual funds. The Fund may invest in emerging markets, but will normally limit such investments to 30% of its net assets, measured at the time of purchase. Due to the nature of the securities in which the Fund invests, it may have relatively high portfolio turnover compared to other funds.

To implement the strategy, the portfolio managers utilize two complementary processes: a “top-down” macro analysis and a “bottom-up” risk/reward analysis, each of which are described below. These processes each employ certain proprietary models which provide forward-looking insights into capital markets and which seek to identify indicators of market stress or potential upside. Such models include a proprietary options implied information model that monitors day-to-day

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movements in options prices for indicators of risk and reward between asset classes, sectors, and regions. Using the proprietary models, the portfolio managers will adjust the Fund’s allocations and the underlying security exposures.

• Top-Down Macro Analysis.  The top-down analysis focuses on how the Fund’s assets will be distributed between the global equity and global fixed-income asset classes, and, at times, the commodity asset class. The portfolio managers monitor expected tail gains and losses across the equity, fixed-income, and commodity asset classes. The portfolio managers intend to periodically adjust the Fund’s asset allocation to mitigate downside risk exposure that is perceived to be elevated and obtain exposure to upside gains. Accordingly, the Fund’s allocation to global equity investments, global fixed-income investments, and, at times, commodity-linked investments, will likely shift periodically to minimize exposure to tail losses and enhance exposure to tail gains. The periodic shifts in the Fund’s asset allocation may significantly impact the Fund’s risk profile.

• Bottom-Up Risk/Reward Analysis.  The bottom-up analysis is designed to identify the underlying security exposures that comprise the Fund’s equity, fixed-income, and commodity asset classes, and periodically rebalance the Fund’s portfolio to maximize exposure to securities that are expected to provide tail gains while minimizing exposure to securities that are expected to provide tail losses. Within the Fund’s equity component, the portfolio managers intend to adjust the portfolio’s sector, currency, and regional exposures away from market capitalization weights based on their evaluation of expected tail loss and gain. Within the Fund’s fixed-income component, the portfolio managers intend to adjust the portfolio’s credit, duration, and regional exposures using the same analysis. Within the Fund’s commodity asset class, the portfolio managers intend to adjust individual commodity or sector exposures when appropriate using the same analysis.

The Fund may invest in equity securities including common and preferred stock, convertible equity securities, sponsored and unsponsored American Depositary Receipts and European Depositary Receipts, non-registered or restricted securities, warrants, and securities of other investment companies. The Fund may invest across all fixed-income sectors, including U.S. and non-U.S. government debt securities (“sovereign debt”). The types of fixed-income securities in which the Fund may invest include asset-backed securities, bank loans, corporate bonds, commercial paper, commercial and residential mortgage-backed securities, mortgage dollar rolls, depositary receipts, and floating-rate securities. The Fund may invest up to 20% of its assets in high-yield/high-risk bonds, also known as “junk” bonds. The Fund’s exposure to commodities may be obtained through ETFs, mutual funds and, at times, exchange-traded notes. In pursuing its investment objective, the Fund will have exposure to investments that are tied economically to a number of countries throughout the world.

The Fund intends to limit its use of derivatives to hedging portfolio risk arising from the Fund’s investments. For example, the Fund may use foreign exchange contracts or other similar instruments to reduce the impact of foreign exchange rate changes on the Fund’s value. In addition, the Fund may use derivatives instruments such as futures to hedge or limit the market exposure when the exchanges or markets in which the securities principally trade are closed or not available, or liquidity is scarce. The Fund may also enter into short sales for hedging purposes.

The Fund may lend portfolio securities on a short-term or long-term basis, in an amount equal to up to one-third of its total assets as determined at the time of the loan origination.

Please retain this Supplement with your records.

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