Japan’s new security legislation, enacted on September 30, 2015 amid fierce debate over its constitutionality, is designed to enable a “seamless response” to any security situation that may arise. While public debate has been fixated on the re-interpretation of Article 9 of the Japanese Constitution, which underpins the theoretical foundation of this new legislation, there are also important international law issues that need to be addressed. After briefly reviewing the historical background leading to the adoption of the new security legislation and its contents, this article examines how the Self-Defense Force (SDF) can respond with the use of force to contemporary security issues within the new legislative framework, while also complying with the relevant rules of international law. It examines three different situations in which the SDF may find itself operating: “gray zone” situations; peacekeeping operations with a mandate to protect civilians; and collective self-defense. It concludes that while the new security legislation goes some way to mend the unravelled seams left by the previous legislative regime, disjuncture between the new legislative regime and relevant rules of international law remains, leaving gaps and uncertainties that can be exploited by hostile actors.