Adolescents spend a substantial portion of their time using social media. Yet, there is a lack of understanding regarding how often parents and adolescents communicate about this social media use. To address this gap, we developed the Parent-Adolescent Communication about Adolescents' Social Media Use Scale (PACAS). In a first data wave, among 388 Dutch adolescents (13-15 years; 54% girls), exploratory and confirmatory factor analyses distinguished four scales: parental solicitation, adolescent disclosure, adolescent secrecy, and parental knowledge. All four scales had strong internal reliability and correlated in the expected directions. We re-established the validity and internal reliability and obtained test-retest reliability in a second wave, in which 330 adolescents were surveyed again. The findings show that parents and adolescents infrequently communicate about social media. Parental knowledge about adolescents' social media use strongly depends on the communication efforts of both parties. Altogether, the PACAS provides a valuable tool to explore the dynamics of parent-adolescent communication about social media. Prior State of Knowledge: Adolescents use social media around the clock. This raises concerns among parents about what their adolescents do and experience on social media. Yet, our understanding of how parents and adolescents communicate about these issues is limited.Novel Contributions: This study developed the Parent-Adolescent Communication about Adolescents' Social Media Use Scale (PACAS), which consists of four scales: parental solicitation, adolescent disclosure, adolescent secrecy, and parental knowledge. The findings show that parents and adolescents infrequently communicate about social media.Practical Implications: The findings emphasize that parents should not only solicit information but also foster open communication so that adolescents can willingly disclose information about their social media use. Only then can parents keep informed about their adolescents' social media experiences.