<p>Chemotherapeutic agents used in cancer treatment cause damage to both oocytes and granulosa cells, resulting in follicle loss and consequently premature ovarian failure. In this study the animals were randomly assigned to three groups: control (<i>n</i> = 6), chemotherapy (<i>n</i> = 6), and stem cell treatment (<i>n</i> = 6). Ovarian failure was induced in the chemotherapy and stem cell groups by intraperitoneal administration of cyclophosphamide (200&#xa0;mg/kg) on days 1 and 8. Ovarian stromal stem cells (OSSCs) were isolated from the ovaries of 4-week-old donor rats (<i>n</i> = 2) using the explant culture method. On day 9, isolated stromal cells were transplanted bilaterally into the ovaries of rats in the stem cell group. Surface markers were analyzed by flow cytometry in OSSCs, and their adipogenic, osteogenic, and chondrogenic differentiation potentials were evaluated under appropriate in vitro conditions. The number of follicles and the morphological features of the ovarian tissue were histologically examined using hematoxylin and eosin (H&amp;E) staining. Caspase 3 expression in ovarian tissue was analyzed using TUNEL and immunohistochemistry methods. Flow cytometry analysis of isolated ovarian stromal cells showed that CD54, CD90 and CD45 surface markers were expressed, while CD29 was expressed at a lower level. These findings confirmed the ability of OSSCs to differentiate into adipogenic, osteogenic, and chondrogenic lineages in vitro. Follicle counting, TUNEL and immunohistochemical analysis showed that treatment with ovarian stromal stem cells significantly reduced the number of atretic follicles and increased the number of normally developing follicles in the ovaries of chemotherapy-treated rats. Our research demonstrates the therapeutic effects of (allogeneic) ovarian stromal stem cells present in their niche on ovarian toxicity induced by chemotherapy, suggesting an alternative treatment option for approaches aimed at preserving fertility.</p>