Owing to the importance of serpentinites for planetary geochemical and geodynamic processes, there has been much work discerning the origins of their parent rocks, including distinguishing between serpentinites derived from a subducting plate versus overlying mantle in exhumed subduction complexes. The island of New Caledonia (SW Pacific Ocean) provides a rare window into Cenozoic Pacific subduction processes. The island is unique in exposing both an exceptionally preserved high‐pressure, low‐temperature subduction complex and one of the largest supra‐subduction zone ophiolites in the world. Previous studies disagree on the origin of serpentinites in the subduction complex. In this study, we analyze 23 serpentinites from this complex for whole‐rock major and trace element geochemistry and stable isotope (δD, δ18O) compositions. Our data reveal two distinct groups of serpentinites: Group I samples in the northern portion of the complex are pervasively serpentinized, and exhibit enriched heavy rare earth element (REE) compositions and δ18O values between +6.7‰ and +10.2‰. In contrast, Group II serpentinites in the south preserve relict orthopyroxene and olivine, and show depleted trace element compositions and comparatively lower δ18O values between +5.1‰ and +8.0‰. We interpret Group I serpentinites to derive from downgoing plate mantle, whereas Group II serpentinites derive from overlying mantle wedge, exhibiting remarkable similarity to the REE geochemistry of the structurally overlying New Caledonia ophiolite. Our results establish the subduction complex in New Caledonia as an unusual natural record of the entrainment and exhumation of mantle from both the overlying mantle wedge and the downgoing plate in an oceanic subduction zone. Plain Language Summary The hydration of Earth's mantle produces rocks called serpentinites that are important to chemical cycling within the Earth system. This process, a form of metamorphism, occurs in several types of tectonic settings on Earth. Serpentinites formed in these different settings are imparted with unique geochemical “fingerprints” due to different types of fluids and varied compositions of the original mantle material. In metamorphic complexes that preserve remnants of subduction zones, serpentinites can offer clues to the original tectonic setting and subsequent mechanics of subduction. We analyze stable isotope and whole‐rock compositions to determine the origin of the mantle that produced serpentinites found in the metamorphic complex preserved in New Caledonia (SW Pacific). We discover two distinct groups of serpentinites: one group in the northern portion of the complex likely formed on the ocean floor prior to subduction, and experienced high degrees of fluid alteration as it was metamorphosed. In contrast, a second group of serpentinites in the southern portion of the complex resembles material from the mantle overlying the subducting plate and is less altered. This locality represents an uncommon example, globally, where material from the overlying mantle was entrained and then preserved at the surface in an exhumed subduction complex. Key Points Two compositionally distinct types of serpentinites are identified in the New Caledonia subduction complex Some serpentinites derive from the subducting plate, others from the overlying mantle wedge New Caledonia preserves evidence for incorporation and exhumation of mantle hangingwall via the subduction channel