This paper explores Sino-Iranian relations in the context of the US Persian Gulf policy after WW II. Based on diplomatic history, using primary Chinese, Iranian, and US sources the paper traces the relations back to the establishment of the Islamic Republic in 1979 until the initiation of the US maximum pressure strategy by the Donald Trump administration after its withdrawal from the Joint Comprehensive Plan of Action in 2018. In analyzing the global impact of the regional security issues in the Persian Gulf, the paper draws on regional security complex theory as well as geopolitical theory in the tradition of Halford Mackinder including the academic debates on the relevance of the theory that has evolved especially among Chinese scholars. The background for the analysis is that Sino-Iranian relations quickly developed after the establishment of the Islamic Republic. China became Iran’s largest trading partner and friend in international affairs. China bought oil, invested in the energy sector, and until 1997 Beijing supported Iran in developing the civil nuclear energy program. During the negotiations up to the deal on Iran’s program with the UN, EU, Germany, and the five permanent members in the UN Security Council, China acted as mediator between Iran and the US. After the US withdrawal from the deal in 2018 China has constantly criticized the US unilateral sanctions but simultaneously put its investments on hold and significantly reduced its trade with Iran. Obviously, the question is whether China is important for Iran in times of sanctions and conflicts with the US. This paper focuses on why Iran is important to China and identifies three areas: energy, economy, and the geopolitical geography of Iran, thus making Iran an important actor in China’s global development policy with the Belt and Road Initiative the kernel in the Chinese geoeconomic and geopolitical strategy. Both China and Iran have criticized the US liberal order and argue for an alternative world order. Taking Iran as a case study, the paper investigates how China is promoting itself diplomatically as an important global player in the security issues in the Persian Gulf by insisting on a multilateral approach. Besides diplomatic history and geopolitical academic debates the paper is based on interviews with Chinese scholars in Beijing and Shanghai and American researchers in Washington, DC.