This paper looks at the little-known history of leftists and Arab nationalists in the Gulf states and the Arabian Peninsula. In our contemporary understanding of the region, the focus on political Islam and religious conflict obscures the fact that the region had a long history of radical politics before the advent of Islamism. The Gulf states all experienced labour movements, strikes and mobilisations by clandestine leftist groups from the 1950s onwards. Transnational connections that linked regional arenas of revolutionary politics such as South Yemen and Dhufar to Kuwait, Bahrain and Saudi Arabia were key for this activism. This paper will outline through which networks these movements mobilised, what their strategic visions and ideological outlook were, and what led to their ultimate defeat. While remnants of these movements still exist, and have a certain amount of influence on politics in Bahrain, Kuwait and to a lesser extent Saudi Arabia and Oman, these movements had lost much of their mass appeal by the late 1970s. This decline happened for a multitude of reasons. Ideological divisions, as well as alliances of opposition groups with often competing regimes led to factionalism. It was also related to their anti-religious outlook and efforts by the regimes to repress and discredit them. In addition, the regional surge of Islamist movements also had an impact. Particularly after 1975, the regimes managed to co-opt many former leftist and Arab nationalist opposition activists and integrate them into the rentier political economy of the Gulf. Nevertheless, particularly in Bahrain, one can still find several leftist parties, who have a strong presence in the labour unions. These parties, as well as the unions, also played a key role in the 1990s intifada and in the uprising since 2011. This paper is based on interviews with current and former leftist activists across the Gulf states and the wider region, a close reading of opposition publications, as well as archival research.