National identity definitions determine who belongs to the national ingroup (e.g., “us Germans”) versus the “foreign” outgroup prone to hostile outgroup bias. We conducted five studies in two countries investigating if viewing the ingroup's national identity as fixed exacerbates the perceived divide between ingroup and outgroup and thus increases anti‐immigrant hostility, while a malleable view blurs the divide and reduces anti‐immigrant hostility. In a Prestudy (58 participants), an Implicit Theory of National Identity Scale was developed. In Studies 1 (154 participants) and 2 (390 participants), our scale predicted individuals’ prejudice and participation rates in a hypothetical referendum and a real petition against immigrants. In Studies 3 (225 participants) and 4 (225 participants), experimental evidence was obtained. Leading participants to believe that the definition of “a true compatriot” changes over time (rather than remaining the same) resulted in lower levels of prejudice and participation rates in an anti‐immigrant petition.