Friendships are significant to social development and offer support in times of transition, such as adolescence and emerging adulthood. However, friendships are not always stable, and dissolutions occur in two forms, complete dissolutions and downgrade dissolutions. Complete dissolutions involve entirely separating ties with a friend and downgrade dissolutions are characterized by remaining friends, but not as close. This study investigated the relationship between experiencing either form of friendship instability and relational aggression at two transitional time points, and whether this relationship was mediated by friendship jealousy. Data were collected from middle school and college samples in the Midwest. Results indicated that friendship instabilities are positively associated with relational aggression. Friendship jealousy fully mediated the relationship between downgrade dissolution and relational aggression in adolescents, and partially mediated the relationship between complete dissolution and relational aggression in emerging adults.