This study examines the impact of the Tax Cuts and Jobs Act of 2017 (TCJA) on domestic labor union bargaining strength faced by U.S. multinational corporations (MNCs). Before TCJA, U.S. MNCs operated under a worldwide tax system that imposed incremental U.S. taxes on earnings repatriated from overseas, prompting MNCs to leave large cash balances abroad while simultaneously allowing them to strategically shelter cash from domestic union demands to preserve their bargaining power. The TCJA resolves such “trapped cash” concerns by allowing U.S. MNCs to repatriate foreign cash without incurring incremental U.S. tax. This allows domestic workers to access previously sheltered foreign cash reserves, thereby strengthening the bargaining power of unions representing domestic workers. Consistent with our prediction, we find that MNCs become more vulnerable to both potential and enacted collective bargaining power of labor unions after TCJA, evidenced by increases in union membership rates and both the frequency and duration of strikes, respectively. In additional analyses using an entropy-balanced sample of MNCs, we provide supporting evidence that TCJA’s “unchaining” of previously trapped foreign cash is the primary channel through which potential union strength increases ex-post.