This paper studies the effects of an historically large federal corporate income tax cut on U.S. firms and workers, leveraging quasi-experimental policy variation from the 2017 law known as the Tax Cuts and Jobs Act. To identify causal effects, we use employer-employee matched federal tax records and an event study design comparing similarly-sized firms in the same industry that faced divergent tax changes due to their pre-existing legal status. Reductions in marginal income tax rates cause increases in sales, profits, investment, and employment, with responses driven by firms in capital-intensive industries. Workers’ earnings gains are concentrated in executive pay and in the top 10% of the within-firm income distribution, while workers in the bottom 90% of the distribution see no change in earnings. Interpreted through the lens of a stylized model, our estimates imply that a $1 marginal reduction in corporate tax revenue generates an additional $0.10 in output, with 78% of gains flowing to the top 10% of the income distribution.