The large-tailed antshrike (Mackenziaena leachii) is a species of bird in subfamily Thamnophilinae of family Thamnophilidae, the "typical antbirds". It is found in Argentina, Brazil, Paraguay, and Uruguay.

The large-tailed antshrike is a large dark antbird with a long full tail. It is 25 to 26 cm (9.8 to 10 in) long and weighs 58 to 62 g (2.0 to 2.2 oz). The species exhibits significant sexual dimorphism, though both sexes have a moderately long black bill with a hook at the end like true shrikes. Adult males are mostly black with small white spots on their crown, nape, upperparts, and sides. Adult females are mostly brownish black. Their forehead is rufous, their crown is rufous with black edges on its feathers, their upperparts have pale buff spots, and their wings have pale buff bars. Their underparts are thickly spotted with buffy white and their crissum is barred with buffy white. Subadults resemble adult females but their spots are larger and their belly and tail have faint pale bars.