Previous studies demonstrated that when individuals interact with outgroup members on social media, in online games, or through (a)synchronous chats, prejudice is reduced. Evaluations of real-world interventions, however, did not consistently confirm the positive impact of online intergroup contact. We advance the literature and investigate whether participation in a global online intergroup contact program predicts lower prejudice as well as increased outgroup knowledge, confidence, and tendencies to take collective action on behalf of outgroup members. We also assess if the quantity of online intergroup contact moderates developments of the outcome measures over time. Applying a pre-post design, participants (N = 547) completed surveys before and after the intervention. One follow-up survey was, depending on the program cohort, administered with a delay of six, 12, and 18 months. Throughout the intervention, prejudice decreased, and collective action tendencies, outgroup knowledge, as well as confidence in one's ability to communicate in intercultural environments increased. These trends were maintained for up to 18 months after program completion. Changes in attitude, knowledge, confidence, or collective action tendencies did not differ systematically between a four-weeks and an eight-weeks module. We conclude that online intergroup contact is a powerful tool to promote harmonious intergroup relations at scale.