An Arabidopsis GH3 gene WES1 encodes an auxin-conjugating enzyme that plays a role in stress responses by modulating endogenous levels of active auxin through a negative feedback regulation. Here, we report a photomorphogenic role for WES1 in hypocotyl growth. Hypocotyls of the WES1-overexpressing wes1-D and the knockout wes1 mutants were similar to control hypocotyls in darkness. However, the wes1-D hypocotyls were significantly shorter but the wes1 hypocotyls were longer than control hypocotyls under red light. Accordingly, WES1 transcription was up-regulated in a phytochrome B mutant. These results provide support for WES1 regulating hypocotyl growth by mediating phytochrome B-perceived light signals.