Light is an essential environmental factor that facilitates the robust upward growth of post-germinative seedlings emerging from buried seeds that is partly mediated by the photoreceptors. Salinity stress hampers plant growth and development and reduces yield. However, the involvement and regulatory role of photoreceptors and light signaling factors to salt stress are largely unknown. Here, we report that mutants of the phytochrome B (phyB) photoreceptor showed reduced sensitivity to salt-inhibited hypocotyl elongation in darkness, and that PHYTOCHROME-INTERACTING FACTOR 3 (PIF3) acts downstream of phyB in regulating this process in Arabidopsis thaliana. We also show that SALT OVERLY SENSITIVE 2 (SOS2) regulates phyB protein accumulation under salt stress in darkness. Surprisingly, salt treatment induces phyB nuclear body formation in darkness. Moreover, we found that the phosphorylation at residue Ser-86 of phyB is essential for its function, and the scaffold protein 14-3-3κ is involved in the regulation of phyB under salt stress in darkness. Taken together, our study reveals a regulatory role of the phyB–PIF3 module in mediating post-germination growth in darkness in response to salt stress.