Abstract Combination of multiple excellent characteristics, such as large surface area, strong acidity, high acid density and good dispersion of active sites, in a mesoporous solid acid catalyst is desirable for achieving superior catalytic activity in production of biodiesel but challenging. Herein, an acidic ionic liquid (IL)-functionalized mesoporous melamine-formaldehyde polymer (MMFP-IL) was prepared for the first time by quaternary ammonization of mesoporous nitrogen-rich melamine-formaldehyde polymer (MMFP) with 1,3-propanesultone, followed by treatment with H3PW12O40 (HPW). The MMFP, containing abundant aminal groups and triazine rings, could be facilely synthesized from lost-cost and readily available monomers (i.e., melamine and paraformaldehyde), which provides massive sites for immobilizing more IL and HPW via chemical post-modification method. The MMFP-IL catalyst was systematically characterized, which was revealed to possess unique and outstanding properties, including abundant mesoporous structures with high specific surface area (283.0 m2/g), high acid concentration (2.2 mmol/g), strong acidity and fine distribution of acid sites. These properties endowed MMFP-IL to have excellent activity in esterification of oleic acid with methanol to produce biodiesel under mild conditions, outperforming those of various solid acids (e.g., commercial strong acidic resins and HPW). In addition, the kinetics and thermodynamics of the reaction were investigated over the MMFP-IL catalyst. More importantly, the MMFP-IL catalyst was robust, heterogeneous, and recyclable without significant deactivation after four cycles, which is attributed to the immobilization of acidic ionic liquid onto the robust MMFP via strong covalent bond.