SummaryRegulations of Arabidopsis seedling growth by two proteins, which belong to different classes of transcription factors, are poorly understood. MYC2 and GBF1 belong to bHLH and bZIP classes of transcription factors, respectively, and function in cryptochrome‐mediated blue light signaling. Here, we have investigated the molecular and functional interrelation of MYC2 and GBF1 in blue light‐mediated photomorphogenesis. Our study reveals that MYC2 and GBF1 colocalize and physically interact in the nucleus. This interaction requires the N‐terminal domain of each protein. The atmyc2 gbf1 double mutant analyses and transgenic studies have revealed that MYC2 and GBF1 act antagonistically and inhibit the activity of each other to regulate hypocotyl growth and several other biological processes. This study further reveals that MYC2 and GBF1 bind to HYH promoter and inhibit each other through non‐DNA binding bHLH–bZIP heterodimers. These results, taken together, provide insights into the mechanistic view on the concerted regulatory role of MYC2 and GBF1 in Arabidopsis seedling development.