The invention describes a watchstrap comprising two separate strap sections, a first section comprising one or more holes in correspondence to which fastening a second section, to fasten the strap and the watch around a wrist. The strap comprises a fastening system with a male part removably fixed to the first section through one of the holes to set a predefined width of the strap, and a female part, fixed on the second section and provided with a receiving seat for snap-engagement of the male part, for an automatic and pre-adjusted fastening of the strap around the wrist, the female part including means for releasing the male part, to open the strap. The invention describes also a fastening system for the strap of a watch and a watch comprising the strap with the corresponding fastening system.