Objective: We aimed to develop an ecological momentary assessment (EMA) measure and sampling protocol to assess the near-term impact of experiences with social media use (SMU) that are associated with risk and protective factors for adolescent suicide. Methods: To develop the EMA measure, we consulted literature reviews and conducted focus groups with the target population, adolescents at risk for suicide. Subsequently, we refined the measure through interviews with experts and cognitive interviews with adolescents, through which we explored adolescents' thought processes as they considered questions and response options. Data were recorded, transcribed, and analyzed using thematic analysis. Results: The initial measure had 37 items assessing a range of harmful and beneficial aspects of SMU. Through expert and cognitive interviews, we refined the measure to 4 pathways assessing positive and negative experiences with SMU as well as positive and negative in-person interactions. Each pathway included a maximum of 11 items, as well as 2 items pertaining to SMU at nighttime to be assessed once daily. Acceptable targets the EMA measure's sampling protocol included a 10-day data collection window with text message-based prompts to complete the measure triggered 2-4 times daily. Conclusions: By assessing a range of risk and protective factors for youth suicide, while using methods to reduce participant burden, we established content validity for the EMA measure and acceptability for the sampling protocol among youth at high risk of suicide.HIGHLIGHTSDevelopment of an ecological momentary assessment measure and sampling protocolExploring brief momentary assessment of social media's impact on adolescent suicidal riskMulti-phase approach to establishing content validity and an acceptable sampling protocol.