Abstract: Device (1) for treating a yarn, comprising a main body (2) axially crossed by a treatment chamber (3) inside which the yarn (F) slides; at least a first side channel (4) in fluid communication with the treatment chamber (3) for the introduction of at least one substance for treating the yarn (F) and at least a second side channel (5) in fluid communication with said chamber (3) for the introduction of compressed air. The device (1) can be inserted within a system (11) for treating the yarn (F). The system (11) comprises at least one conduit (6) for supplying the device with the substance to be used for the treatment, a conduit (13) for introducing compressed air into the device (1) and a discharge conduit (12) for exiting the air and any fibre possibly detached from the yarn (F).