To assess the prognostic significance of immunoglobulin (Ig) paraproteinaemia in mucosa‐associated lymphoid tissue (MALT) lymphoma, 218 patients diagnosed with MALT lymphoma were enrolled in this study. Serum Ig paraprotein was detected in 42 of 218 patients (19.3%), mostly IgM‐K (15, 35.7%), followed by IgM‐L and IgG‐L. Advanced age (p = 0.025), poor Eastern Cooperative Oncology Group performance status (p = 0.014), bone marrow involvement (p = 0.019), B symptoms (p = 0.039), advanced disease stage (III−IV) (p < 0.0001), elevated serum β2‐microglobulin level (p < 0.0001), multiple extranodal sites of involvement (p < 0.0001), nodal involvement (p < 0.0001), systemic therapy (p < 0.0001) and higher MALT‐lymphoma International Prognostic Index (MALT‐IPI) scores (p = 0.001) were significantly associated with the presence of serum Ig paraprotein. Multivariate Cox regression analysis showed that Ig paraproteinaemia was an independent prognostic predictor for inferior progression‐free survival (PFS) and overall survival. A new prognostic index based on MALT‐IPI and Ig paraproteinaemia, as assessed using receiver operating characteristic curves and the area under the curve statistics, showed better discriminative ability than MALT‐IPI in predicting PFS. In conclusion, Ig paraproteinaemia was a promising prognostic predictor for MALT lymphoma. Ig paraproteinaemia together with MALT‐IPI might contribute to optimising therapeutic management in clinical practice.