ImportanceLesbian, gay, bisexual, transgender, queer, and/or questioning (LGBTQ+) youth face worse mental health outcomes than non-LGBTQ+ peers. Family support may mitigate this, but sparse evidence demonstrates this in clinical settings.ObjectivesTo compare depression and suicide risk between LGBTQ+ and non-LGBTQ+ youth in primary care settings and to investigate whether family support mitigates these negative mental health outcomes.Design, Setting, and ParticipantsThis cross-sectional study uses data from well care visits completed by adolescents aged 13 to 19 years from February 2022 through May 2023, including the Patient Health Questionnaire–9 Modified for Teens (PHQ-9-M) and the Adolescent Health Questionnaire (AHQ; an electronic screener assessing identity, behaviors, and guardian support), at 32 urban or suburban care clinics in Pennsylvania and New Jersey.ExposuresThe primary exposure was self-reported LGBTQ+ status. Family support moderators included parental discussion of adolescent strengths and listening to feelings. Race and ethnicity (determined via parent or guardian report at visit check-in), sex, payer, language, age, and geography were covariates.Main Outcomes and MeasuresPHQ-9-M–derived mental health outcomes, including total score, recent suicidal ideation, and past suicide attempt.ResultsThe sample included 60 626 adolescents; among them, 9936 (16.4%) were LGBTQ+, 15 387 (25.5%) were Black, and 30 296 (50.0%) were assigned female sex at birth. LGBTQ+ youth, compared with non-LGBTQ+ youth, had significantly higher median (IQR) PHQ-9-M scores (5 [2-9] vs 1 [0-3]; P < .001) and prevalence of suicidal ideation (1568 [15.8%] vs 1723 [3.4%]; P < .001). Fewer LGBTQ+ youth endorsed parental support than non-LGBTQ+ youth (discussion of strengths, 8535 [85.9%] vs 47 003 [92.7%]; P < .001; and listening to feelings, 7930 [79.8%] vs 47 177 [93.1%]; P < .001). In linear regression adjusted for demographic characteristics and parental discussion of strengths, LGBTQ+ status was associated with a higher PHQ-9-M score (mean difference, 3.3 points; 95% CI, 3.2-3.3 points). In logistic regression, LGBTQ+ youth had increased adjusted odds of suicidal ideation (adjusted odds ratio, 4.3; 95% CI, 4.0-4.7) and prior suicide attempt (adjusted odds ratio, 4.4; 95% CI, 4.0-4.7). Parental support significantly moderated the association of LGBTQ+ status with PHQ-9-M score and suicidal ideation, with greater protection against these outcomes for LGBTQ+ vs non-LGBTQ+ youth.Conclusions and RelevanceCompared with non-LGBTQ+ youth, LGBTQ+ youth at primary care visits had more depressive symptoms and higher odds of suicidal ideation and prior suicide attempt. Youth-reported parental support was protective against these outcomes, suggesting potential benefits of family support–focused interventions to mitigate mental health inequities for LGBTQ+ youth.