指数基金（英语：Index Fund）或指数型基金，是消极管理投资基金的主要形式，有时候被用来指代所有的消极管理投资基金。从广义上讲，ETF（交易所交易基金）也属于指数基金。

指数基金的投资理念是在证券市场上选定一部分符合条件的证券，这些证券可以通过客观标准（如：总资本，总股本，成交量，主营业务）选定，也可以通过主观标准(如成长性，被市场低估的程度)选定；被选定的证券共同构成一个指数，每一个证券都拥有一个确定的权重（即该证券在整个投资组合中所占的比例），指数基金经理按照这个指数购买证券，创建一个与指数完全相同或基本相同的投资组合，这样就创造了一只指数基金。

An index fund (also index tracker) is a mutual fund or exchange-traded fund (ETF) with specific rules of construction that are adhered to regardless of market conditions. An index fund’s rules of construction clearly identify the type of companies suitable for the fund. The most commonly known index fund, the S&P 500 Index Fund, is based on the rules established by S&P Dow Jones Indices for their S&P 500 Index. Equity index funds would include groups of stocks with similar characteristics such as the size, value, profitability and/or the geographic location of the companies. A group of stocks may include companies from the United States, Non-US Developed, Emerging Market or Frontier Market countries. Additional index funds within these geographic markets may include indexes of companies that include rules based on company characteristics or factors, such as companies that are small, mid-sized, large, small value, large value, small growth, large growth, the level of gross profitability or investment capital, real estate, or indexes based on commodities and fixed-income. Index funds have also been designed to include rules that screen for social and sustainable criteria. Companies are purchased and held within the index fund when they meet the specific index rules or parameters and are sold when they move outside of those rules or parameters. Think of an index fund as an investment utilizing rules-based investing. Those rules may also include trading or implementation rules, such as tax-management, tracking error minimization, large block trading or patient/flexible trading strategies that allows for greater tracking error, but lower market impact costs. Some index providers announce changes of the companies in their index before the change date and other index providers do not make such announcements.