The aim of this study was to evaluate whether chronic endometritis (CE) and uterine endometrium microbiota were associated with repeated implantation failures (RIFs) and recurrent pregnancy losses (RPLs). In this prospective study, uterine endometrial specimens were obtained from 24 women with RIF, 27 with RPL, and 29 fertile control women. Immunohistochemical staining of CD138 for CE and 16S ribosomal RNA (rRNA) sequencing analysis for uterine endometrium microbiota were performed simultaneously. To assess CE, Liu’s method, McQueen scores and plasma cell count/10 mm² were used. The frequency of CE (plasma cells > 5.15/10 mm²) was higher in women with RPL (29.6%) than in fertile controls (6.8%, p < 0.05). The plasma cell count/10 mm² in women with RPL (median 1.53, range 0–252.6, p < 0.01) and women with RIF (median 0.6, range 0–6.98, p < 0.05) was higher than in fertile controls (median 0, range 0–29). The uterine endometrium microbiota in women with RPL or RIF was not significantly different from that in fertile controls. However, the relative dominance rate of Lactobacillus iners (median 4.7%, range 0–99.9 vs. median 0%, range 0–100, p < 0.001) and the positive rate of Ureaplasma species (36.3% vs. 8.6%, p < 0.05) were higher in 11 women with CE than in 69 women without CE. The results suggest that CE may be involved in the pathophysiology of RPL and RIF. Lactobacillus iners and Ureaplasma species may be associated with the etiology of CE.