Tumor-associated macrophages (TAMs) shape tumor immunity and therapeutic efficacy. However, it is poorly understood if and how post-translational modifications (PTMs) intrinsically affect the phenotype and function of TAMs. Here, we found that peptidylarginine deiminase 4 (PAD4) manifested the highest expression among common PTM enzymes in TAMs and negatively correlated to clinical response to immune checkpoint blockade (ICB). Genetic and pharmacological inhibition of PAD4 in macrophages prevented tumor progression in tumor-bearing mouse models, accompanied by an increase in macrophage MHC-II expression and T-cell effector function. Mechanistically, PAD4 citrullinated STAT1 at arginine 121 (R121), thereby promoting the interaction between STAT1 and PIAS1; and the loss of PAD4 abolished this interaction, ablating the inhibitory role of PIAS1 in the expression of MHC-II machinery in macrophages and enhancing T-cell activation. Thus, the PAD4-STAT1-PIAS1 axis is a previously unknown intrinsic immune restriction mechanism in macrophages and may serve as a cancer immunotherapy target.