A thread aspirating device (1) for circular knitting machines, comprising a body (2) of the thread aspirating device having a lower side (3), defining an inlet opening (4) enabling inlet of one or more threads in the thread aspirating device, and an up per side (5), provided with an engaging portion (6) delimiting a passage opening (7) in the upper side. The thread aspirating device comprises a mouthpiece (10) extending between an open lower end (11), defining a mounting portion (12) complementarily shaped to the engaging portion, and an upper end (13) provided with an aspirating opening (14). The mouthpiece is superiorly mounted on the body of the device so that the mounting portion corresponds to the engaging portion; the mouthpiece and the body internally define an aspirating space (20) of the threads between the inlet opening and the aspirating opening. The mouthpiece and the body overall form, in mounting condition, a thread aspirating device in a single body.