The present invention relates to an extensible device (1) for a watch strap (40) of predefined length, in particular for a diving watch strap. The extensible device can adopt two configurations, one short and one long, by a relative movement of two extreme parts (2, 3) between a mutually close position and a mutually distant position. Locking means (22, 24, 27, 32) are provided to keep the devicein its short configuration, and the latter also has at least one control member (11). The latter can be used to operate the locking means in order to release the extreme parts from each other and allow the short configuration to be changed to the long configuration. The control member (11) is designed to be movable between two positions, a first position in which it is close to the extreme parts,and a second pulled position, in which it is remote from the extreme parts.; The first position corresponds to a locked condition of the locking means, while the second position corresponds to their unlocked condition, which ensures a high degree of safety of the locking means and limits the risk of accidental manipulation of the extensible device.