National flags embody their nations’ core values and beliefs but they may be perceived differently in today’s increasingly polarized societies. We conducted two studies ( N = 768) in the United States to examine partisan differences between Democrats and Republicans, and a third study in the United Kingdom ( N = 509) between supporters of the Labour and Conservative parties. Across all three studies, we consistently found that Democrat and Labour participants (compared to their Republican and Conservative counterparts) associated their respective national flags with less positive and more negative affect and concepts. Studies 2 and 3 further showed that these partisan divides were driven by how much people associated their national flags with their political outgroup. For Democrats and Labour participants, associating their national flags with their political outgroup predicted less positive and more negative affect, but associating the flags with their political ingroup predicted more positive and less negative affect.