Soybean oils having less unsaturation than conventional soybean oil (SBO) were studied as sustainable plasticizers for ethylene-propylene-diene monomer (EPDM) rubber compounds vulcanized with peroxide. In EPDM formulations, part of the petroleum-based paraffinic oil (PO) was replaced with soy-derived plasticizers such as high oleic soybean oil (HOSO), partially hydrogenated soybean oil (PHSO), and completely hydrogenated soybean oil (HSO). The mechanical and viscoelastic properties of EPDM rubbers formulated with soy-derived plasticizers were studied and compared with the properties of PO-based rubber. Torque rheometry was used to study the vulcanization process and the properties of the vulcanized rubber compounds were evaluated using tensile testing, compression testing, dynamic mechanical analysis, and solvent swelling experiments. The equilibrium swelling test and examination of gel fraction demonstrated the incorporation of soy-based process aids into the rubber network during vulcanization. It was found that soy-derived plasticizers could partially substitute for the petroleum-based plasticizer in EPDM compounds with minimal impact on properties. An additional benefit of the use of hydrogenated SBO is the increase of crosslink density in vulcanized rubber indicating enhanced peroxide efficiency.