We present Mo isotopic ratios of molybdenite from five porphyry molybdenum deposits (Chagele, Sharang, Jiru, Qulong, and Zhuonuo) and one quartz-molybdenite vein-type deposit (Jigongcun) along the Gangdese metallogenic belt in the Tibetan Plateau. These deposits represent a sequence of consecutive events of the India-Asia collision at different periods. Additional molybdenite samples from the Henderson Mo deposit (USA), the oceanic subduction-related El Teniente (Chile), and Bingham (USA) porphyry Cu-(Mo) deposits were analyzed for better understanding the controls on the Mo isotope systematics of molybdenite. The results show that molybdenite from Sharang, Jiru, Qulong, and Zhuonuo deposits have similar δ⁹⁷Mo (∼0 ‰), in agreement with the values of the Henderson Mo deposit (−0.10 ‰). In contrast, samples from the Changle and Jigongcun deposit have δ⁹⁷Mo of 0.85 ‰ to 0.88 ‰ and −0.48 %, respectively. Molybdenite from the El Teniente and Bingham deposits yields intermediate δ⁹⁷Mo of 0.27 and 0.46 ‰, respectively. The Mo isotopes, combined with Nd isotope data of the ore-bearing porphyries, indicate that source of the ore-related magmas has fundamental effects on the Mo isotopic compositions of molybdenite. Our study indicates that molybdenite related to crustal-, and mantle-derived magmas has positive or negative δ⁹⁷Mo values, respectively, whereas molybdenite from porphyries formed by crust-mantle mixing has δ⁹⁷Mo close to 0 ‰. It is concluded that the Mo isotope composition in the porphyry system is a huge source signature, without relation to the tectonic setting under which the porphyry deposits formed.