Mutational signatures reflect common patterns based on the counts of mutations and their sequence context. The prognostic value of these signatures, mirroring various carcinogenetic processes of cancers, are unexplored in gastrointestinal cancers. Our aim was to evaluate possible prognostic relevance of mutational signatures in gastrointestinal carcinomas after adjusting with the traditional prognostic factors.We used publicly available data from The Cancer Genome Atlas and Pan-Cancer Analysis of Whole Genomes to evaluate the associations between survival endpoints and activity of mutational signatures in seven types of gastrointestinal cancers.Most strikingly, the high activity of age-related single-base substitution 5 (SBS5) and SBS40 signatures were in rectal adenocarcinomas associated with both improved overall survival (OS) [for SBS5 hazard ratio (HR) 0.130; 95% CI=0.03-0.56, for SBS40 HR=0.072; 95% CI=0.012-0.44, respectively] and similarly also to rectal cancer-specific survival. In patients with left-sided (but not right-sided) colon adenocarcinoma, the high activity of SBS2 signatures, formed due to APOBEC activity, predicted shortened OS. In pancreatic cancer, the high activity of SBS10b, caused by polymerase epsilon exonuclease proofreading defects, was associated both with longer OS (HR=0.44; 95% CI=0.205-0.96) and pancreatic cancer-specific survival (HR=0.32; 95% CI=0.112-0.91).Several mutational signatures seem to have clinically meaningful, cancer-specific associations with prognosis among gastrointestinal cancers.