Extranodal marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue (MALT) is the third most prevalent subtype of mature B-cell lymphomas. While MALT lymphoma typically has a favorable prognosis, the outlook can worsen dramatically if histological transformation (HT) occurs, usually into a more aggressive form like diffuse large B-cell lymphoma (DLBCL). Large-scale study with long-term follow-up specifically focused on the transformation of MALT lymphoma is limited. This study utilized the SEER database to analyze 20,944 patients diagnosed with MALT lymphoma from 2000 to 2020. We also included a cohort of de novo DLBCL patients for comparison. Our findings showed that HT occurred in 1.59% of MALT lymphoma cases, with 5- and 10-year cumulative incidences of 1.2% and 2.2%, respectively. Patients presenting with B symptoms, age ≥ 60, and advanced-stage disease were at a significantly higher risk for HT. The median time to transformation was 3.67 years. Both univariate and multivariate analyses demonstrated that age ≥ 60 was a significant adverse factor for lymphoma-specific survival (LSS) and overall survival (OS) following HT. This population-based study contributes to the characterization of transformation risk and outcomes in MALT lymphoma and may help inform clinical surveillance strategies.