Updated project metadata. Macrophages are effector cells of the innate immune system and undergo phenotypical changes in response to organ injury and repair. They are most often classified as proinflammatory M1 and anti-inflammatory M2 macrophages. Protein arginine deiminase (PAD) enzymes, that catalyze the conversion of protein-bound arginine into citrulline, an irreversible posttranslational modification, are expressed in macrophages. However, the substrate scope of the PADs and their role in the immune cells remain not well defined. This study aims to investigate the role of PADs in the THP-1 macrophage polarization to M1 and M2 phenotype and identify the citrullinated proteins, and the modified Arg that are associated with this biological switch using mass spectrometry. Our study showed that PAD2, and to a lesser extent PAD1 and PAD4 were dominantly expressed in M1 macrophages. We showed that inhibiting PADs by BB-Cl-amidine decreased the macrophage’s polarization to M1 and increased to M2 phenotype. The process was mediated by the downregulation of proteins involved in NF-kβ pathway. Silencing of PAD2 confirmed activation to M2 macrophages through upregulation of the antiviral innate immune response and interferon signaling. 192 novel citrullination sites that belong to inflammation, cell death and DNA/RNA processing pathways were identified in M1 and M2 macrophages.