This study investigated the combined effects of photoperiod and dietary P level on bone osteoclast and osteoblast activity, and morphology, as well as plasma vitamin D status in Atlantic salmon post-smolts. The fish were reared under continuous light (LL) or 12 h light/dark (LD) per day, and fed diets with insufficient phosphorous (P) (4 g kg−1 available dietary P, LP) or sufficient P (8 g kg−1 available dietary P, HP) for 79 days in seawater. LL significantly increased plasma 25(OH)D3 level and bone tartrate resistant acid phosphatase (TRACP) activity, and decreased bone collagen XI α1 chain (col11a1) mRNA transcription. The LP diet significantly increased plasma 1,25(OH)2D3 level and bone alkaline phosphatase activity, but decreased bone TRACP activity and matrix metalloproteinase 13 (mmp13) mRNA transcription, and vertebral mineral content, stiffness and length/dorso-ventral diameter (l/d) ratio. A significant interaction between light and P was only observed on the l/d ratio, and the LP-LL group was the only group that developed vertebrae with a compressed morphology (significantly lowest l/d ratio). In practical terms, these results show that Atlantic salmon post-smolts under continuous light may need P supplemented diets to support normal bone development.