Objectives: Psoriasis is a common chronic inflammatory skin disease characterized by keratinocyte hyperproliferation and an excessive inflammatory response. Agents that can attenuate keratinocyte hyperproliferation and excessive inflammatory responses are considered potentially useful for the treatment of psoriasis. Daehwangmokdanpitang (DHMDPT) exhibits a broad range of bioactivities, including anti-proliferative and anti-inflammatory effects. This study aims to evaluate the anti-psoriatic potential of DHMDPT in vitro. Methods: HaCaT keratinocytes were stimulated with a mixture of IL-17A, IL-22, oncostatin M, IL-1α, and TNF-α (M5) to establish an in vitro psoriatic keratinocyte model. Cell viability was measured using the MTT assay. Quantitative real-time PCR (qRT-PCR) was performed to measure the mRNA levels of the hyperproliferative marker gene keratin 6 (KRT6) and inflammatory factors such as IL-6, TNF-α, and IL-23A. Additionally, chemokines including CCL5, CCL2, CCL20, and CXCL1 were measured by qRT-PCR. Results: DHMDPT attenuated M5-induced hyperproliferation, as indicated by a reduction in KRT6 expression in HaCaT keratinocytes. M5 stimulation significantly upregulated the mRNA levels of IL-6, TNF-α, and IL-23A. However, DHMDPT treatment attenuated the upregulation of IL-6 but not TNF-α or IL-23A. Additionally, DHMDPT inhibited the expression of CCL5, CCL2, and CXCL1, but not CCL20. Conclusion: DHMDPT effectively attenuated the M5-induced proliferation and inflammatory response in HaCaT keratinocytes. Therefore, DHMDPT could be an attractive candidate for future development as an anti-psoriatic agent.