Although chemotherapy can stimulate antitumor immunity by inducing interferon (IFN) response, the functional role of tumor-associated macrophages in this scenario remains unclear. Here, we found that IFN-activated proinflammatory macrophages after neoadjuvant chemotherapy enhanced antitumor immunity but promoted cancer chemoresistance. Mechanistically, IFN induced expression of cytoplasmic long noncoding RNA IFN-responsive nuclear factor-kappa B activator (IRENA) in macrophages, which triggered nuclear factor-kappa B signaling via dimerizing protein kinase R and subsequently increased production of protumor inflammatory cytokines. By constructing macrophage-conditional IRENA-knockout mice, we found that targeting IRENA in IFN-activated macrophages abrogated their protumor effects, while retaining their capacity to enhance antitumor immunity. Clinically, IRENA expression in post-chemotherapy macrophages was associated with poor patient survival. These findings indicate that lncRNA can determine the dichotomy of inflammatory cells on cancer progression and antitumor immunity and suggest that targeting IRENA is an effective therapeutic strategy to reversing tumor-promoting inflammation. Song and colleagues report that in breast cancer, chemotherapy induces the IRENA lncRNA, which reprograms tumor suppressive macrophages to protumorigenic phenotypes and promotes chemoresistance.