How much of the U.S. stock market is owned by foreigners?
Through the end of the second quarter of 2018, the most recent period for which the data is available from the U.S. Treasury Department, the answer is $8.1 trillion, which is the equivalent of about 35% of the $23.0 trillion market capitalization of the S&P 500 at that time.
In 2002, foreigners owned just $1.4 trillion, or the equivalent of 15% of the S&P 500's market cap at that time. In between, the share of U.S. equities held by foreigners rose steadily to peak at 36.5% in 2015, but has dipped slightly and stabilized in recent years.
If you think about it, the growth of foreign holdings of U.S. stocks has had considerable influence over the growth of the U.S. stock market during the early 21st century.
References
Standard and Poor. S&P 500 Earnings and Estimate Report. [Excel Spreadsheet]. 18 July 2019.
U.S. Department of the Treasury and Federal Reserve Bank of New York. Foreign Portfolio Holdings of U.S. Securities, as of June 30, 2008. [PDF Document]. 18 November 2009.
U.S. Department of the Treasury and Federal Reserve Bank of New York. Foreign Portfolio Holdings of U.S. Securities, as of June 30, 2012. [PDF Document]. April 2013.
U.S. Department of the Treasury and Federal Reserve Bank of New York. Foreign Portfolio Holdings of U.S. Securities, as of June 29, 2018. [PDF Document]. 15 May 2019.