Passive source localization is an important issue as there are numerous applications including search and rescue and public safety. Hybrid passive source localization combining time difference of arrival (TDOA) and received signal strength difference (RSSD) is investigated in this paper. First, a TDOA and RSSD based linear weighted least-squares (WLS) estimator for passive source localization is presented. Second, the maximum likelihood (ML) estimator and Cramer-Rao lower bound (CRLB) for the hybrid localization scheme are derived as performance benchmarks. Numerical results are presented which demonstrate that the proposed hybrid localization method provides significantly better accuracy than using only the TDOA.