Abstract Seed storage protein (SSP) acts as one of the main components of seed storage reserves, of which accumulation is tightly mediated by a sophisticated regulatory network. However, whether and how gibberellin (GA) signaling is involved in this important biological event is not fully understood. Here, we show that SSP content in Arabidopsis (Arabidopsis thaliana) is significantly reduced by GA and increased in the GA biosynthesis triple mutant ga3ox1/3/4. Further investigation shows that the DELLA protein RGA-LIKE3 (RGL3), a negative regulator of GA signaling, is important for SSP accumulation. In rgl3 and 35S:RGL3-HA, the expression of SSP genes is down- and upregulated, respectively, compared with that in the wild-type. RGL3 interacts with ABSCISIC ACID INSENSITIVE3 (ABI3), a critical transcription factor for seed developmental processes governing SSP accumulation, both in vivo and in vitro, thus greatly promoting the transcriptional activating ability of ABI3 on SSP genes. In addition, genetic evidence shows that RGL3 and ABI3 regulate SSP accumulation in an interdependent manner. Therefore, we reveal a function of RGL3, a little studied DELLA member, as a coactivator of ABI3 to promote SSP biosynthesis during seed maturation stage. This finding advances the understanding of mechanisms in GA-mediated seed storage reserve accumulation.