The object of the present invention is to provide a method for producing an .alpha.,.beta.-unsaturated carboxylic acid in higher selectivity. The present invention resides in a method for producing an .alpha.,.beta.-unsaturated carboxylic acid from an olefin or an .alpha.,.beta.-unsaturated aldehyde in liquid phase by using a noble metal-containing catalyst and by causing a compound (A) having an acid dissociation exponent (pKa) of less than 4 to be present in the liquid phase.