Myeloid cells co-expressing the markers CD11b, Ly-6G, and SiglecF can be found in large numbers in murine lung adenocarcinomas and accelerate cancer growth by fostering tumor cell invasion, angiogenesis, and immunosuppression; however, some of these cells' fundamental features remain unexplored. Here, we show that tumor-infiltrating CD11b⁺ Ly-6G⁺ SiglecF^high cells are bona fide mature neutrophils and therefore differ from other myeloid cells, including SiglecF^high eosinophils, SiglecF^high macrophages, and CD11b⁺ Ly-6G⁺ myeloid-derived suppressor cells. We further show that SiglecF^high neutrophils gradually accumulate in growing tumors, where they can live for several days; this lifespan is in marked contrast to that of their SiglecF^low counterparts and neutrophils in general, which live for several hours only. Together, these findings reveal distinct attributes for tumor-promoting SiglecF^high neutrophils and help explain their deleterious accumulation in the tumor bed.