Cyclin D1 is an important regulator of cell cycle progression. Phosphorylation of cyclin D1 at Thr286 by GSK3β triggers its nuclear export and cytoplasmic proteolysis via the 26S proteasome. Cyclin D1 overexpression is a common event in various types of human cancers; however, reports of mutations are extremely rare. We analyzed mutations of the cyclin D1 gene, CCND1, in 88 endometrial cancer tissue specimens and detected mutations in 2 cases (2.3%). Both were unreported mutations with substitution of threonine to isoleucine at codon 286 (T286I). These two tumors harbored coexisting mutations in K-ras, PIK3CA and/or PTEN and showed accumulation of cyclin D1 in the nucleus by immunohistochemistry. Furthermore, we analyzed the functions of mutant cyclin D1 (T286I) by luciferase assays, immunofluorescence, western blotting and clonogenic cell survival assays in HEK-293T cells. We found that exogenous mutant cyclin D1 (T286I) accumulated in the nuclei in HEK-293T cells, and that it inhibited the expression of pRb. Additionally, the number of colonies was increased by introduction of mutant cyclin D1 (T286I) compared to that of wild-type cyclin D1. In conclusion, we identified an unreported CCND1 mutation (T286I) in two endometrial cancers and revealed that the mutation was functional for inducing cell proliferation in human cells.