The present invention provides a process for producing 2-hydroxy-4-methylthiobutyric acid comprising the following steps (1) to (4): (1): hydrolyzing 2-hydroxy-4-methylthiobutyronitrile in the presence of sulfuric acid, (2): separating the reaction mixture obtained in the step (1) into an oil layer containing 2-hydroxy-4-methylthiobutyric acid and a water layer, (3): concentrating the oil layer obtained in the step (2), and (4) heating the concentrated product obtained in the step (3).