Soybean cv. Zaoshu18 has been identified as an effective source resistant to Phytophthora root rot of soybean. The objectives of this study were to identify and map the Phytophthora resistance gene in Zaoshu18 in order to facilitate its utilization and marker-assisted breeding. Zaoshu18 was crossed with susceptible cultivar Williams to develop mapping populations. Genetic analysis of F2 and F2:3 progenies indicated that resistance to Phytophthora sojae in Zaoshu18 was controlled by a dominant single gene. The resistance gene was temporarily designated as RpsZS18. Based on linkage analysis of SSR markers, RpsZS18 was located on soybean molecular linkage group (MLG) D1b between SSR markers Sat_069 and Sat_183 with genetic distances 10.0 cM and 8.3 cM, respectively. This is the first locus for Phytophthora resistance that has been identified on MLG D1b.