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Nanoparticles of intermetallic compounds can function as high surface area catalysts with unique activity and selectivity that emerge from their atomically ordered crystal structures. High-entropy intermetallic nanoparticles are especially interesting catalytic targets, as their complex compositions are anticipated to further modify and tune catalytic properties. However, the synthesis of high-entropy intermetallic nanoparticles has been challenging, since simultaneous control of crystal structure and multimetal mixing is often favored at high temperatures while nanoscopic particle sizes are favored at lower temperatures. Here, we demonstrate a solution-based postsynthetic transformation strategy that converts high-entropy alloy nanoparticles into high-entropy intermetallics. Colloidal high-entropy alloy nanoparticles with predefined homogeneous metal mixing, including NiPdPtRhIr, NiFeCoPdPt, and NiFePdPtIr, convert to γ-brass, CsCl, and NiAs-type high-entropy intermetallics of Zn, In, and Sn, respectively, which span a range of compositions and crystal structures. The γ-brass (NiPdPtRhIr)10Zn42 nanoparticles are active catalysts for acetylene semi-hydrogenation, exhibiting better selectivity than isostructural Pd10Zn42 and pointing to the role of high-entropy mixing in improving catalytic performance.