Endometrial cancer (EC) is increasingly affecting younger women, and progesterone therapy remains the cornerstone of fertility-preserving treatment (FPT). However, progesterone resistance presents a major clinical challenge. While the tumor microbiome and its metabolites have been implicated in various cancers, their role in EC and treatment resistance remains poorly understood. In this study, we comprehensively characterized the intratumor microbiota of 72 early-stage EC patients undergoing FPT using 16S rRNA sequencing. We found that patients who were sensitive to medroxyprogesterone acetate (MPA) had significantly higher alpha and beta diversity of intratumor microbiota compared to resistant patients. At the genus level, several butyrate-producing bacteria, including Butyrivibrio, Clostridium, and Faecalibacterium, were enriched in the MPA sensitive group. Targeted qPCR and metabolomic analyses confirmed the presence of these bacteria and elevated butyrate levels in tumor tissues. Functionally, butyrate suppressed tumor cell proliferation and enhanced MPA sensitivity both in vitro and in vivo. Mechanistically, butyrate promoted ferroptosis by downregulating CISD1. Butyrate-induced CISD1 inhibition enhanced iron accumulation, ROS generation, GPX4 expression, and HMGB1 release, thereby promoting ferroptosis. Overexpression of CISD1 partially rescued the ferroptosis of butyrate, confirming its role in mediating progesterone sensitivity. In addition, butyrate upregulated progesterone receptor expression, which further supported its role in sensitizing EC cells to progesterone therapy. This study is the first to demonstrate the critical role of the EC tumor microbiome and its metabolite butyrate in modulating progesterone response in FPT. Our findings provide a new mechanistic insight and offer a strong rationale for targeting the microbiota-ferroptosis axis to overcome progesterone resistance in EC patients.