Previous research indicates that partner responsiveness and self-expansion play key roles in the creation, maintenance, and improvement of close relationships. This experiment examined the hypothesis that active (vs. passive) partner support for an individual’s opportunity for self-expansion would increase relationship satisfaction. In an experimental task manipulated to be either self-expanding or stressful, dating couple members (N = 116; 58 couples) received active or passive support messages, ostensibly from their partners. Among those in longer term (14–60 months), but not in shorter term relationships, relationship satisfaction increased significantly more for those who received active (vs. passive) support for self-expansion. This same pattern was not found when partners’ messages responded to a stressful task or for couples in short-term relationships. This study provides the first experimental evidence for effects on relationship satisfaction of partner support for individual self-expansion. In addition, the findings suggest the potential substantial importance of relationship length for moderating self-expansion processes.