Clinicopathologic information of gastrointestinal (GI) lymphoma in Southeast Asia is lacking. A retrospective analysis of 120 cases of GI lymphoma in Thailand diagnosed at Siriraj Hospital based on WHO classification was performed. All were non-Hodgkin lymphoma (NHL). The peak age was in the sixth and seventh decades; a slight male preponderance was observed. Sites of involvement included stomach (49.2%), intestine (46.7%), and multiple sites (4.2%). There were 104 cases of primary GI lymphoma (86.7%) and 16 cases of secondary GI lymphoma (13.3%). Presenting GI symptoms were more common in the former; while superficial lymphadenopathy and fever were more common in the latter. Mass lesions were observed in both groups (72.1% vs 56.3%). Localized and advanced diseases were found in 68.3% and 31.7% of primary GI lymphomas, respectively. The most common type of lymphoma in both groups was diffuse large B-cell lymphoma. Lymphoepithelial lesions (LEL) were not significantly different between the two groups (58.2% vs 42.9%), but Helicobacterpylori infection was significantly associated with primary gastric lymphoma (p < 0.0001). The treatment of choice for localized primary GI lymphoma is controversial. Complete surgical resection may increase the chance of complete remission, but mortality and relapse rates might be higher than those observed with combination chemotherapy alone. GI lymphomas in Thailand are mostly primary B-cell NHL. LEL is not indicative of primary GI lymphoma, but H. pylori infection is closely associated with primary gastric lymphoma. A prospective study to determine the treatment of choice for localized GI lymphoma is needed.