Japan plays an integral role in the US–China relationship, as a major trading partner of both and the third-largest economy in the world. Both the United States and China have deep ties with Japan, and both states have used their relationship with Japan to seek to advance their interests with regard the other. Japan’s power is shrinking relative to the United States and China, but it remains an important regional and global actor – capable of substantially enhancing US power with regard to China and of acting as a spoiler for both US and Chinese interests. This makes both the US–China and Japan–China relationships in many ways a triangular US–Japan–China relationship, with the added challenges that a three-way relationship entails. When considering the challenges of the US–China relationship moving forward, careful attention should be paid to the impact of Japan on the evolving US–China dynamic.