The present invention relates to a downhole swivel sub (100) for insertion in a string, the downhole swivel sub comprising : • - a first body (101) comprising a first end (103) and a second end (104); • - a second tubular body (102) enclosing the second end of the first body, the first body and the second body forming a bore (106); • - a coupling member (107) which, in a first position, engages the first body with the second body, and in a second position, disengages the second body from the first body; the said coupling member being fully enclosed in the second body and forming a chamber with the inner surface of the second body, said chamber being in fluidic communication with the external of the downhole swivel sub through a vent in the second body for allowing the movement of the coupling member from its first position towards its second position.