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Para-toluic acid, systematically known as 4-methylbenzoic acid is a substituted benzoic acid in which a methyl group occupies the position directly opposite the carboxylic acid group on the benzene ring. It is one of three positional isomers of toluic acid and it holds the greatest industrial significance among them due to its central role as an intermediate in terephthalic acid production.

Chemically, para-toluic acid belongs to the aromatic carboxylic acid family and can be understood structurally as toluene with one hydrogen replaced by a carboxylic acid group at the para position, or equivalently, as benzoic acid with an added methyl substituent. It appears as a white to slightly yellow-cream (occasionally yellow-brown) crystalline solid, and its high degree of molecular symmetry — the two substituents sitting directly across the ring from one another — allows it to pack more efficiently into a crystal lattice than its ortho or meta counterparts, giving it the highest melting point of the three isomers. para-Toluic acid arises in industrial terephthalic acid manufacturing both directly from partial oxidation of para-xylene and from the catalytic hydrogenation of 4-carboxybenzaldehyde (4-CBA), making it a key process intermediate whose removal from the mother liquor is essential to producing high-purity terephthalic acid.