Estrogen receptor-positive breast cancer remains difficult to treat, driving the search for new therapeutic agents. Juncus effusus L., a medicinal plant rich in bioactive constituents, has shown preliminary anticancer activity, yet its efficacy in hormone-responsive tumor models in vivo has not been established. We evaluated the antitumor effects of the ethyl acetate sub-extract of J. effusus L. subsp. effusus in an N-methyl-N-nitrosourea (NMU)-induced rat mammary tumor model, assessing tumor burden, histopathology, and molecular markers. Thirty-six Sprague–Dawley rats were divided into six groups (n = 6): sham, control, reference, and three groups receiving the sub-extract at 100, 200, or 400 mg/kg. After NMU induction (50 mg/kg), treatment began once tumor volumes reached approximately 2000 mm³ and continued for eight weeks. The 200 mg/kg dose markedly reduced tumor volume relative to control (p < 0.0001) and lowered both ERα and PCNA expression (p < 0.001 and p < 0.0001). The 100 and 200 mg/kg groups also showed lower p53 and ERα mRNA levels than the other treatment groups. HIF-1α, though elevated in control tumors, declined only modestly and without statistical significance after treatment. These results indicate that J. effusus subsp. effusus, particularly at 200 mg/kg, suppresses proliferation and modulates ERα- and p53 expression, supporting its potential for further preclinical investigation.