Dedifferentiation of retinal pigment epithelium (RPE) cells and choroidal neovascularization (CNV) contributes to the pathogenesis of age-related macular degeneration (AMD). MicroRNAs (miRNAs) have crucial roles in AMD onset and progression. We thus aim to investigate the effects of miRNAs on RPE dedifferentiation and endothelium cell (EC) behavior, and analyze its downstream pathways. We have previously identified miR-302d-3p as the most downregulated miRNA signature along with RPE differentiation. Herein, in vitro study supported that miR-302d-3p induces RPE dedifferentiation typified by reduction of RPE characteristic markers, interrupts its phagocytosis, and promotes its migration, proliferation, and cell-cycle progression. c-Jun was identified as a potential upstream transcript factor for MIR302D, which might modulate RPE function by regulating miR-302d-3p expression. P21Waf1/Cip1, a cyclin-dependent kinase inhibitor encoded by the CDKN1A gene, was identified as a downstream target of miR-302d-3p. Our data suggested that p21Waf1/Cip1 could promote RPE differentiation, and inhibit its proliferation, migration, and cell-cycle progression. We also demonstrated that miR-302d-3p suppresses RPE differentiation through directly targeting p21Waf1/Cip1. In addition, the miR-302d-3p/CDKN1A axis was also involved in regulating tube formation of ECs, indicating its potential involvement in CNV formation. Taken together, our study implies that miR-302d-3p, regulated by c-Jun, contributes to the pathogenesis of both atrophic and exudative AMD. MiR-302d-3p promotes RPE dedifferentiation, migration, proliferation and cell-cycle progression, inhibits RPE phagocytosis, and induces abnormal EC behavior by targeting p21Waf1/Cip1. Pharmacological miR-302d-3p inhibitors are prospective therapeutic options for prevention and treatment of AMD.