Background: Rhododendron arboreum and Rhododendron campanulatum are two important trees of Uttarakhand, known for their ethnopharmacological importance. In the present study, the aqueous and methanolic extracts of leaves of both the plants were analyzed for phytochemical, antioxidant, and antiproliferative activities against cancer cell line and repression of vascular endothelial cell growth factor (VEGF) and hypoxia-inducible factor-1 (HIF-1α) transcription. Materials and Methods: Aqueous and methanolic leaf extracts of both the plants were prepared through pressurized liquid extraction method. Various assays were performed to analyze phytochemical and antioxidant potential. The antiproliferative activity was determined through 2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxy anilide salt (XTT) assay. Further, the effect of plant extracts on the modulation of HIF-1α and VEGF mRNA expression was analyzed through quantitative real-time polymerase chain reaction. Results: A significantly high total phenol content, total flavonoid content, free radical scavenging, and reducing power activities were observed in both the plant extracts. High-performance thin-layer chromatography analysis indicated the presence of ascorbic acid, quercetin, gallic acid, and hesperidin in different extracts. XTT reduction assay confirmed the antiproliferative activities of aqueous extracts against cancer cell line, HeLa. Aqueous extract of R. arboreum (RAA), at a concentration of 31.25 μg/ml, inhibited 60.12% and 25.41% proliferation of HeLa and Vero cell lines, respectively. An equal concentration of aqueous extract of R. campanulatum (RCA) inhibited the growth by 48.04% and 15.17% for HeLa and Vero cell lines, respectively. Further, the RAA and RCA have downregulated the expression of two key angiogenic factors responsible for tumor neovascularization, HIF-1α and VEGF to a great extent. Conclusion: The present study affirms that the leaf extracts of R. arboreum and R. campanulatum have enormous potential to be developed as an effective natural antioxidant and anticancer drug.