IntroductionDebate about the precise role of social media use (SMU) in the mental health of today's adolescents is still ongoing. The present study adds to the literature by focusing on adolescents' experiences during SMU and in their offline activities through the lens of basic psychological needs, which are central to self‐determination theory.MethodsTo examine the joint and supplementary effects of need‐experiences in the offline and SMU domain (i.e., SMU satisfaction, offline satisfaction, SMU frustration, offline frustration) on several indicators of adolescents' mental health (i.e., vitality, life satisfaction, sleep quality, anxiety, and depression), polynomial regression analyses were used. Three cross‐sectional samples were collected in Belgium, including early to mid‐adolescents during the COVID‐pandemic (Sample 1; N = 447; Mage = 14.26; 54.4% female) as well as postpandemic (Sample 2; N = 179, Mage = 15.25; 54.2% female), and among college students in postpandemic times (Sample 3; N = 4977; Mage = 20.72; 69.1% female).ResultsThe results showed that need‐experiences common to both domains were a robust factor associated with mental health. This finding was obtained across all samples and outcomes, with need satisfaction playing a beneficial and need frustration a harmful role. We further found that offline need‐experiences serve as a more crucial predictor of adolescents' mental health than need experiences on social media.ConclusionsAdolescents' experiences of need satisfaction and need frustration on social media and in offline activities are central to their mental health. However, only relying on SMU as a single source of need satisfaction may not be recommended, given that offline experiences seem to be more decisive.