Romantic relationship involvement has repeatedly been associated with the incidence of externalizing behavior problems, but little is known about the nature and developmental significance of this relation. The current study extends previous research by investigating whether and through what processes romantic relationships distinctively predict externalizing behavior problems during adolescence compared to emerging adulthood. Data came from the Minnesota Longitudinal Study of Parents and Children. As predicted, higher levels of romantic relationship security at 16 years were associated with lower levels of externalizing behavior problems during both adolescence and emerging adulthood, but this inverse relation was stronger for emerging adults than for adolescents. This relation was not attributable either to earlier quality of family and peer relationships or emerging adulthood competence. Thus, security of romantic relationships may become increasingly predictive of individual differences in externalizing behavior problems as individuals move from adolescence to emerging adulthood.