Mantle source heterogeneity of arc‐related magmas was traditionally thought to be predominantly affected by slab‐derived components. However, the role of forearc serpentinites in causing subarc mantle heterogeneity remains poorly constrained. Here, we present the first Zn isotope data for lavas from the Mariana and Ryukyu subduction zones. Notably, the δ⁶⁶Zn values of basaltic lavas from the Mariana arc (0.16–0.18‰) and southern Okinawa Trough (0.15–0.17‰) are generally lower (∼0.10‰) than those of the mid‐ocean ridge basalts (MORB) (0.27 ± 0.05‰). Since mantle melting and magmatic differentiation respectively induce heavy Zn isotope enrichment in primary and evolved magmas, while melt extraction yields the limited Zn isotope fractionation in the mantle, lavas with low δ⁶⁶Zn values thus potentially indicate the addition of subducted components. The negative correlation between the δ⁶⁶Zn values and the Ba/La ratios of the basaltic lavas suggests the involvement of isotopically light fluids in their mantle sources. Forearc serpentinites are typically characterized by extremely light Zn isotope compositions. Such forearc materials were likely dragged downward to subarc depths and released isotopically light Zn in fluids to modify the overlying mantle wedge, thereby producing low δ⁶⁶Zn values in arc‐related magmas. Beyond subarc depths, forearc serpentinites are broken down completely, so light Zn isotope fluids are absent. Accordingly, the basalts from the middle Mariana and Okinawa Trough display MORB‐like δ⁶⁶Zn values. Collectively, Zn isotope provides new insights into the role of the recycling of forearc serpentinites in generating chemical heterogeneity in the mantle source of subduction‐related magmas.