Previous studies have suggested that adolescent mothers with higher social support have lower depressive symptoms. This is a longitudinal study of adolescent mothers to examine the association of social support and depressive symptoms over one year postpartum. This was a prospective study of adolescent mothers (N at baseline = 120, N at 1 year = 89; age < 19 years) enrolled in a teen tot program. Participants completed the Center for Epidemiological Studies Depression Scale for children (CES-DC) and the Duke-UNC Functional Social Support Questionnaire at baseline, 12 weeks, and 1 year. A score of ≥ 16 on the CES-DC was suggestive of major depression. The mean CES-DC scores of the adolescent mothers were ≥16 points at all three time points (baseline: mean = 18.7 ± 10.3; 53% ≥ 16; 12 weeks: mean = 18.4 ± 11.4, 57% ≥ 16; one year: mean = 20.0 ± 11.4; 57% ≥ 16). Social support had a significant, inverse association with depressive symptoms for all participants from baseline to 12 weeks with a stronger association for those with more depressive symptoms (score ≥ 16) at baseline (beta = −0.030 ± 0.007; P < 0.001) than for those with fewer depressive symptoms (score < 16) at baseline (beta = −0.013 ± 0.006; P = 0.021). From 12 weeks to one year, increased social support was only significantly associated with decreased depressive symptoms for those with a higher baseline level of depressive symptoms (beta = − 0.039 ± 0.009; P < 0.001). Depressive symptoms were prevalent among adolescent mothers. For more depressed adolescent mothers, higher levels of social support were associated with less depressive symptoms over the 1 year follow-up. Effective long-term interventions are needed to lessen depression and enhance social support.