Evidence for the assumption that cannabis use is associated with depression and depressive symptoms is inconsistent and mostly weak. It is likely that the mixed results are due to the fact that prior studies ignored the moderating effects of an individual's genetic vulnerability. The present study takes a first step in scrutinizing the relationship between cannabis use and depressive symptoms by taking a developmental molecular–genetic perspective. Specifically, we concentrated on changes in cannabis use and depressive symptoms over time in a simultaneous manner and differences herein for individuals with and without the short allele of the 5-hydroxytryptamine (serotonin) transporter gene-linked polymorphic region (5-HTTLPR) genotype. Data were from 310 adolescents over a period of 4 years. We used a parallel-process growth model, which allows co-development of cannabis use and depressive symptoms throughout adolescence, and the possible role of the 5-HTTLPR genotype in this process. We used data from the younger siblings of these adolescents in an attempt to replicate potential findings. The parallel-process growth model shows that cannabis use increases the risk for an increase in depressive symptoms over time but only in the presence of the short allele of the 5-HTTLPR genotype. This effect remained significant after controlling for covariates. We did not find conclusive support for the idea that depressive symptoms affect cannabis use. These findings were replicated in the sample of the younger siblings. The findings of the present study show first evidence that the links between cannabis use and depressive symptoms are conditional on the individual's genetic makeup.