A green sulfonated carbon-based catalyst was successfully synthesized through sulfonation of incompletely carbonized coffee residue (SCAC catalyst). The sulfonation temperature was investigated and the catalytic activity was tested via esterification of caprylic acid. SCAC-200, the highest caprylic acid conversion at 4 h (71.5%) and initial TOF based on SO3H acid sites, was synthesized under a carbonization temperature of 600 °C for 4 h and sulfonation temperature of 200 °C for 18 h. Sulfonation temperature plays a dominant role in determining the SO3H site density of SCAC catalysts because side reactions