Macrophage polarization is accompanied by drastic changes in L-arginine metabolism. Two L-arginine catalytic enzymes, iNOS and arginase 1, are well-characterized hallmark molecules of classically and alternatively activated macrophages, respectively. The third metabolic fate of L-arginine is the generation of creatine that acts as a key source of cellular energy reserve, yet little is known about the role of creatine in the immune system. Here, genetic, genomic, metabolic, and immunological analyses revealed that creatine reprogrammed macrophage polarization by suppressing M(interferon-γ [IFN-γ]) yet promoting M(interleukin-4 [IL-4]) effector functions. Mechanistically, creatine inhibited the induction of immune effector molecules, including iNOS, by suppressing IFN-γ-JAK-STAT1 transcription-factor signaling while supporting IL-4-STAT6-activated arginase 1 expression by promoting chromatin remodeling. Depletion of intracellular creatine by ablation of the creatine transporter Slc6a8 altered macrophage-mediated immune responses in vivo. These results uncover a previously uncharacterized role for creatine in macrophage polarization by modulating cellular responses to cytokines such as IFN-γ and IL-4. Copyright © 2019 Elsevier Inc. All rights reserved.