Freedom-of-navigation operations (FONOPs) led by the United States have become a prominent policy tool in the South China Sea. These operations, however, have caused much confusion and consternation in the region despite the limited legal purpose that they officially serve. Why? This article departs from existing research by examining FONOPs as a form of signalling. Utilising an original dataset on US FONOPs in the South China Sea, we explain why and how FONOPs are ambiguous signals. Because of the nature of FONOPs using warships, the complexities in the maritime environment, and the irregularity of FONOPs, serious sender–receiver gaps emerge in the South China Sea regarding whether FONOPs signal resolve, coercive intent, and/or intentions to check certain states’ broader ambitions. Our analysis reveals that some signalling interpretations of FONOPs in the region are more plausible than others, thus helping to reduce the signalling ambiguity of these operations.