Some authors have treated sooty and Rothschild's swifts as conspecific.

The upper side of the wing is mostly blackish brown and the underside somewhat lighter.

[6] The sooty swift is found from southern Minas Gerais in southeastern Brazil south to Rio Grande do Sul, eastern Paraguay, and northern Argentina's Misiones Province.

[6] The sooty swift's breeding season varies somewhat across its range, but is between October and February overall.

It makes a cup or crescent shaped nest of moss, ferns, leaves, pebbles, and mud on a rock face.

The sooty swift's flight calls have been described as "a rapid, rattling ti-i-i-i-a-a-a, as well as tiju-tiju and a weak sip or sip-sip.