We measure market integration at a firm-level for the US stock market with the rest of the world. The properties of firm-level integration are explored across time and industries and then stocks are sorted into high- and low-integration portfolios. The role of the least globally integrated US stocks in domestic and international portfolio diversification is assessed. We show that these stocks can statistically and economically augment diversification potential, especially after 2000. The same stocks can be combined with the US market index to create a portfolio that performs at least as well as an international index portfolio in terms of risk-adjusted returns and tail risk.