Decidual stromal cells (DSCs) are involved in immunoregulatory mechanisms that prevent fetal rejection by the mammalian maternal immune system. Recent studies using single-cell RNA sequencing demonstrated the existence of different types of human and mouse DSCs, highlighting corresponding differentiation (decidualization) pathways, and suggesting their involvement in the immune response during normal and pathological pregnancy. DSCs may be considered tissue-specialized fibroblasts because both DSCs and fibroblasts share phenotypic and functional similarities in immunologically challenged tissues, especially in terms of their immune functions. Indeed, fibroblasts can setup, support, and suppress immune responses and these functions are also performed by DSCs. Moreover, fibroblasts and DSCs can induce ectopic foci as tertiary lymphoid structures (TLSs), and endometriosis, respectively. Thus, understanding DSC immunoregulatory functions is of timely relevance.