Despite their proliferation, limited knowledge exists regarding possible benefits of brief mindfulness ecological momentary interventions (MEMIs) for social anxiety disorder (SAD). Propositions that MEMIs could alleviate SAD symptoms and related clinical outcomes remain untested. This trial evaluated a 14-day MEMI for SAD. Participants with self-reported SAD were randomized to MEMI (n = 96) or self-monitoring app (SM; n = 95). Whereas MEMI instructed mindfulness exercises, SM prompted only self-monitoring five times daily for 14 days. Participants completed state-level self-reports of depression, anxiety, and mindfulness pre-post-mindfulness practice and SAD symptoms, worry, depression severity, repetitive negative thinking, and trait mindfulness at pre-randomization, post-intervention, and 1-month follow-up (1MFU). Hierarchical linear modeling was conducted. The MEMI yielded statistically significantly larger improvements in momentary depression, anxiety, and mindfulness (Cohen's d = -0.10-0.11). Although no between-group effects emerged in alleviating SAD fear and avoidance, excessive worry, depression severity, repetitive negative thinking, and trait mindfulness (-0.13-0.15), within-group effects were significantly small-to-large from pre-post and pre-1MFU (-4.62-0.67). A significant reduction in depression severity occurred in MEMI (-0.63--0.60) but not SM (-0.31--0.29). Brief MEMI and SM yielded nondifferent sustained effects on SAD, comorbid symptoms, and risk factors, highlighting its potential value within stepped-care delivery settings.