An adjustable collar or harness (10) for pets that comprises a strap (11) provided with flaps fitted with reciprocal coupling means, and a ring (12) or similar designed to be attached to a spring clip of a lead, wherein the means for coupling the ends of the strap (11) consist of at least one toothed tab or tape (13) designed to couple with at least one spring pushbutton (or a lever) (17) which grips one or more teeth of the toothed tape (13) allowing the automatic blocking and release of the toothed tape, when appropriately operated.
