© 2020, Springer Science+Business Media, LLC, part of Springer Nature. Eutectic modification through minor addition of boron was firstly proposed to improve the malleability of brittle eutectic high-entropy alloys (EHEAs). The proposed approach involves adding small amounts of eutectic modifier, boron (0.2–1.6 at.%), to a brittle dual-phase [face centred cubic (FCC) and σ phase] Fe35Ni25Cr25Mo15 EHEA. Here, we report the role of boron additions in improvement of the mechanical properties of a Fe35Ni25Cr25Mo15 EHEA. It was found that with the increase in boron additions up to 1 at.%, the lamellar eutectic structure consisting of FCC and σ intermetallic phase gradually transited to a dendrite-like eutectic structure. As a result, the compressive fracture strain of the alloy was increased about 3 times with a slight reduction in strength. The mechanisms of eutectic morphology transition induced by boron additions in the Fe35Ni25Cr25Mo15 EHEA and their effects on mechanical properties were studied through microstructural characterization and thermodynamic analyses. The transition of eutectic morphology from lamellar eutectic to dendrite eutectic is believed to be resulted from the increased constitutional undercooling caused by B additions.The solute redistribution of B in the liquid ahead of the solid/liquid interface resulted in a B concentration gradient, which led to the formation of a constitutionally undercooled zone at the front of the σ phase, which destabilized the coupled growth conditions necessary for the formation of the lamellar eutectic structure. As a result, the eutectic morphology was transformed from lamellar eutectic to a dendritic-like eutectic structure. This type of structure exhibits a lower fraction of phase boundaries leading to improved malleability of the brittle EHEA. This insight can be used to design new advanced EHEAs through adjustment of the eutectic morphology.