Abstract A CoCrFeNiMnPd eutectic high-entropy alloy was designed and prepared. A unique eutectic-dendritic solidification pattern was observed in the as-cast alloy. Heretofore, this kind of pattern was rarely reported no matter in binary or multi-component alloys. Through a series of experimental characterization and theoretical analysis, it could be concluded that the sluggish diffusion effect of high-entropy alloys makes the interface deviate considerably from the local non-equilibrium condition and the alternate arrangement of CoCrFeNiPd-rich FCC and Tetragonal Mn7Pd9 lamellae results in weak interface energy anisotropy, thus forming the seaweed eutectic-dendrite. Application of the designing rules of high-entropy alloys to the current work indicates that mere consideration of the weighted average physical properties of constituent elements is not sufficient to the design of high-entropy alloys with a single solid-solution phase.