Genetic alterations and their association with clinicopathological features in colorectal mucinous carcinoma (MC) remain unknown. In particular, little is known about the mutational status of the BRAF gene, which is activated by oncogenic Ras. This study aimed to evaluate the status of BRAF together with K-ras, p53 and mismatch-repair deficiency to clarify their association with tumorigenesis of colorectal MC. BRAF and K-ras mutations were determined in 43 colorectal MCs by direct sequencing. p53 alteration was investigated immunohistochemically. The status of mismatch-repair deficiency was assessed by microsatellite analyses and immunohistochemistry for hMLH1. We also examined the association between these molecular alterations and clinicopathological features including histological configuration. BRAF mutation was detected in 4 (9.3%) tumors and was located at codon 599 of exon 15 in all cases. K-ras mutation was detected in 13 tumors (30.2%). No BRAF and K-ras mutations were identified simultaneously in the same tumor. The incidence of mismatch-repair deficiency tended to be higher in MC with BRAF mutation than without. In terms of histological configuration, we classified the cases according to growth type by tumor edge morphology. All MCs with BRAF mutation and 9 of 13 MCs (69.2%) with K-ras mutation were classified as polypoid type. BRAF and K-ras mutation did not affect patient prognosis, but non polypoid type was significantly more aggressive than polypoid type. Our findings indicate that BRAF mutation plays an important role in the tumorigenesis of colorectal MC and in tumor edge morphology, similar to K-ras mutation.