Despite the fact that many adolescents spend much time on the Internet, it is unknown who engages in sexually related online activities (SROA) and how these affect adolescent sexual development. The present longitudinal study on 323 adolescents (51.1% girls) aimed to explore how peer attachment processes predicted both SROA and offline sexual behaviors at the age of 17, while also considering puberty and prior offline sexual experiences in order to elucidate potential similarities or differences. Findings based on hierarchical, binary logistic regression analyses revealed that SROA were predicted by alienation attachment to peers (OR=3.36, p<0.05), puberty (OR=1.03, p<0.05), and prior SROA (OR=0.56, p<0.001), while only previous offline sexual experiences at the age of 15 increased the likelihood of offline sexual behaviors at the age of 17 (OR=6.04, p<0.001). Study findings indicate that the Internet provides an additional context for acquiring sexual experiences during adolescence.