Temperature gradient is inevitable in solder joints in advanced electronic packaging; it induces thermomigration (TM), critically affecting the reliability of micro solder joints. Although a FeCoNiMn alloy (FCNM) can resist thermal aging in solder joints, its TM resistance has seldom been examined. Thus, this study investigated the TM in Cu/Sn3.5Ag (SA3.5)/FCNM solder joints between a cold end (50 °C) and a hot end (200 °C) at different times. A multielement intermetallic compound (MEIMC), (Cu, Mn, Fe, Co, Ni)6Sn5, is formed at the SA3.5/FCNM interface in the reflowed and aged solder joints. A (Fe, Co, Ni, Mn)Sn2 MEIMC also formed, but it was very thin. The (Cu, Mn, Fe, Co, Ni)6Sn5 was completely decomposed by TM from the hot-end FCNM. In contrast, the (Cu, Mn, Fe, Co, Ni)6Sn5 MEIMC thickened as the rapid TM flux of Cu from the hot-end Cu cap; however, the low chemical flux from FCNM hindered its formation, which was replaced by Cu6Sn5 formation near the Cu/SA3.5 interface. The findings demonstrate that FCNM providing a very low chemical flux is a remarkably promising diffusion barrier for TM in micro solder joints. •Excellent suppression of thermomigration in Cu/Sn3.5Ag/FeCoNiMn high-entropy alloy.•Very low chemical flux provided by FeCoNiMn to resist thermomigration.•Demonstrating a novel diffusion barrier of FeCoNiMn against thermomigration.