RELMα is a small, secreted protein expressed by type 2 cytokine-activated "M2" macrophages in helminth infection and allergy. At steady state and in response to type 2 cytokines, RELMα is highly expressed by peritoneal macrophages, however, its function in the serosal cavity is unclear. In this study, we generated RELMα TdTomato (Td) reporter/knockout (Rα) mice and investigated RELMα function in IL-4 complex (IL-4c)-induced peritoneal inflammation. We first validated the RELMα transgenic mice and showed that IL-4c injection led to the significant expansion of large peritoneal macrophages that expressed Td but not RELMα protein, while RELMα mice expressed RELMα and not Td. Functionally, RELMα mice had increased IL-4 induced peritoneal macrophage responses and splenomegaly compared to RELMα mice. Gene expression analysis indicated that RELMα peritoneal macrophages were more proliferative and activated than RELMα macrophages, with increased genes associated with T cell responses, growth factor and cytokine signaling, but decreased genes associated with differentiation and maintenance of myeloid cells. We tested the hypothesis that Rα macrophages drive aberrant T cell activation using peritoneal macrophage and T cell co-culture. There were no differences in CD4 T cell effector responses when co-cultured with RELMα or RELMα macrophages, however, RELMα macrophages were impaired in their ability to sustain proliferation of FoxP3 regulatory T cells (Treg). Supportive of the results, immunofluorescent staining of the spleens revealed significantly decreased FoxP3 cells in the RELMα spleens compared to RELMα spleens. Taken together, these studies identify a new RELMα regulatory pathway whereby RELMα-expressing macrophages directly sustain Treg proliferation to limit type 2 inflammatory responses.