Multiple myeloma is a frequent hematological malignancy. Although progress has been made in therapeutic strategies, the prognosis of multiple myeloma is far from satisfactory. Therefore, it is imperative to investigate the precise mechanism of multiple myeloma progression. Lysine Demethylase 4C (KDM4C) was demonstrated to be a vital regulator in cancers, while its action on multiple myeloma remains elusive. Thus, we aimed to investigate the effect of KDM4C on multiple myeloma progression and explored the precise mechanism of action. In this study, 70 multiple myeloma patients and 45 normal donors (volunteers) were enrolled. Results showed that KDM4C was highly expressed in plasma of 70 multiple myeloma patients and multiple myeloma cells. Knockdown of KDM4C suppressed proliferation and migration of multiple myeloma cells. Besides, JAG1 expression was enhanced in plasma of 70 myeloma patients and multiple myeloma cells. JAG1 expression was positively correlated with KDM4C expression. Furthermore, KDM4C knockdown suppressed Notch signaling proteins Notch-1, NICD-1, and Hes-1 in multiple myeloma. Moreover, KDM4C knockdown suppressed the proliferation and migration of multiple myeloma cells through down-regulating JAG1 expression. Collectively, KDM4C promotes the proliferation and migration of multiple myeloma cells by up-regulating JAG1 gene expression. KDM4C may be a promising target for multiple myeloma therapy.