Recently, sexual health scholars have expressed concerns regarding adolescents’ use of sexually explicit internet materials (SEIM) during the COVID-19 pandemic. Therefore, using latent growth curve modeling, the current study explored adolescents’ changes in the frequency of SEIM use before, during, and after a strict lockdown period was established in Belgium. Attention was given to individual differences (i.e., gender, gender identity, sexual orientation, pubertal timing, and sensation seeking). A three-wave panel study over a 15-month period among 522 adolescents was used (Mage = 15.36, SD = 1.51, 67.1% girls). In general, SEIM use did not significantly increase over a 15-month period in the context of the COVID-19 pandemic. Only gender predicted a change in SEIM use frequencies with girls showing a greater, increasing change of SEIM use than boys. When addressing why adolescents used SEIM during a strict lockdown period, sexual arousal, stress, and boredom regulation motivations emerged as the most prevalent motivations. Loneliness regulation was the least prominent motivation. Individual differences were found regarding the gratifications sought according to adolescents’ gender, pubertal timing, and sensation seeking. The findings offer a response to sexual scholars’ worries in terms of adolescents’ SEIM use during the COVID-19 pandemic.