Plants continuously monitor environmental conditions (such as light and temperature) and adjust their growth and development accordingly. The transcription factor PHYTOCHROME-INTERACTING FACTOR4 (PIF4) regulates both light and temperature signaling pathways. Here, we identified ENHANCED PHOTOMORPHOGENIC2 (EPP2) as a new repressor of photomorphogenesis in red, far-red, and blue light. Map-based cloning revealed that EPP2 encodes the SEUSS (SEU) transcription regulator. The C terminus of SEU has transcriptional activation activity, and SEU physically interacts with PIF4. Moreover, SEU promotes the expression of many genes, including auxin biosynthetic and responsive genes, and regulates IAA levels in plants. SEU associates with the regulatory regions of INDOLE-3-ACETIC ACID INDUCIBLE6 (IAA6) and IAA19 in a PIF4-independent manner, whereas the binding of PIF4 to these genes requires SEU. Furthermore, mutations in SEU affect H3K4me3 methylation at IAA6 and IAA19, and SEU positively regulates warm temperature-mediated hypocotyl growth together with PIF4. Collectively, our results reveal that SEU acts as a central regulator integrating light and temperature signals to control plant growth by coordinating with PIF4.