Isonicotinic acid or pyridine-4-carboxylic acid is an organic compound with the formula C5H4N(CO2H). It is a derivative of pyridine with a carboxylic acid substituent at the 4-position. It is an isomer of picolinic acid and nicotinic acid, which have the carboxyl group at the 2- and 3-position respectively compared to the 4-position for isonicotinic acid.
Isonicotinic acids is a term loosely used for derivatives of isonicotinic acid. Hydrazide derivatives include isoniazid, iproniazid, and nialamide. Amide and ester derivatives include ethionamide and dexamethasone isonicotinate.