This article examines the relationship between the U.S.-led Proliferation Security Initiative (PSI) and the 1982 United Nations Convention on the Law of the Sea (UNCLOS). It attempts to answer the questions of whether the PSI is legal or illegal under UNCLOS and whether U.S. accession to UNCLOS would enhance or create difficulties for the implementation of the PSI. The author concludes that U.S. accession to the Convention would not affect adversely the implementation and effectiveness of the PSI. On the contrary, accession to UNCLOS could help increase U.S. credibility and leadership in dealing with the threat to international peace and security posed by weapons of mass destruction proliferation. It also suggests that all the relevant information needs to be gathered and examined carefully in order to answer the question of whether a PSI interdiction action is legal under UNCLOS or not.