Myanmar hosts three north–south trending ophiolite belts, widely interpreted as remnants of the Tethyan Ocean. The Mogok peridotites, situated on the southern margin of Myanmar’s Eastern Ophiolite Belt, have sparked debate regarding their oceanic origin due to their atypical ophiolitic characteristics. In this study, we present Re–Os isotopic and platinum-group element data for nineteen spinel harzburgite samples from the Mogok ultramafic body, in conjunction with recently reported whole-rock major element compositions, to constrain their origin and tectonic setting. The Mogok peridotites have subchondritic ¹⁸⁷Os/¹⁸⁸Os ratios (0.11764–0.12437), with the lowest value yielding a Re-depletion model age of 1.8 Ga. This age estimate suggests their derivation from the Paleoproterozoic or more ancient lithospheric mantle. Moreover, the Mogok peridotites exhibit enrichment in iridium-group platinum group elements (Os, Ir, and Ru) and depletion in palladium-group platinum group elements (Pt and Pd). This geochemical signature closely resembles that of cratonic peridotites, indicating substantial melt extraction during their formation, consistent with their highly refractory bulk major element compositions. These results support a subcontinental lithospheric mantle origin for the Mogok peridotites, rather than previous interpretations invoking Tethys-related crustal cumulates or ophiolitic and sub-arc mantle sources. This implies limited north–south extension of Myanmar’s Eastern Ophiolite Belt, potentially terminating north of the Mogok area. These ancient Mogok peridotites were most likely uplifted from the lithospheric mantle beneath the Sibumasu terrane during the Eocene–Oligocene lithospheric extension following the West Myanmar Block–Sibumasu collision. Our study reveals ancient Gondwana-derived mantle fragments preserved within Southeast Asia’s young collisional belts, a consequence of Tethyan continental rift–drift–collision. This advances our understanding of the region’s tectonic evolution and lithospheric architecture.