Studies of lithospheric mantle from (ultra)slow spreading ridges have shown that melt extraction at mid-ocean ridges may be incomplete, producing metasomatism/refertilization of the shallow lithospheric mantle. However, it remains unclear whether similar processes operate off axis and could affect the cooling lithosphere. Here, we report the discovery of a garnet xenocryst in a petit-spot lava sampled on the top of the downgoing Pacific plate in front of Japan. The trace-element composition of this garnet xenocryst, in particular the low chromium, excludes a peridotitic origin, while the flat mid– to heavy rare earth element pattern does not support direct crystallization from melt percolating through the oceanic lithosphere. Garnet formation is therefore interpreted as formed by a subsolidus reaction of a plagioclase-bearing cumulate during the progressive off-axis cooling of the lithosphere. Combining lithosphere cooling models and the specific physical conditions required for subsolidus formation of garnet in tholeiitic systems (0.7–1.2 GPa) indicates that melt percolation to produce plagioclase-bearing cumulate occurs >150 km off axis. These conditions support that some low-degree melts produced off axis are not collecting to form mid-oceanic ridge basalt (MORB), but percolate and crystallize during the cooling and thickening of the lithospheric mantle. The demonstration of mantle refertilization affecting Pacific lithosphere off axis is critical because such a process could explain the presence of metasomatic domains with distinct physical and chemical properties in the depleted oceanic lithosphere.