Canine gastrointestinal sarcomas, a group of tumours that includes leiomyosarcomas (LMSAs), gastrointestinal stromal tumours (GISTs) and other rarer sarcomas, comprise about 10–30% of all gastrointestinal tumours. This study aims to characterize the histologic characteristics and clinical behaviour in order to identify prognostic factors predictive of outcome. A single institution database search for surgically treated gastrointestinal sarcomas yielded 47 cases with adequate tissue remaining for histologic analysis and 42 cases available for analysis of clinical outcome. Tumours were then prospectively evaluated for mitotic count, necrosis, haemorrhage and inflammation, as well as categorized via immunohistochemical (IHC) staining for smooth muscle actin, c‐KIT and DOG‐1. IHC analysis defined 32 tumours as GISTs, 14 as LMSAs and one as a sarcoma not otherwise specified. For both GISTs and LMSAs, the overall median survival time (MST) is 1024 days (range 31–1456), which did not differ statistically between tumour types (p = .92). The overall metastatic rate of GISTs in this study was 32.1% (n = 9) which was not significantly different to that of LMSAs at 15.3% (n = 2, p = .45). In multivariate analysis, mitotic count under 9 in GIST patients and complete surgical excision in all tumour types correlated with improved MST. For patients with GISTs, the intensity of c‐KIT staining also correlated positively with survival, with an MST of 250 days in cases with weak staining and an MST of 1418 days in cases with moderate or strong c‐KIT staining (p = .005).