1. Safety device for a portable long-barrelled firearm or pneumatic weapon (10), said weapon (A) being gripped by a user with both hands and comprising: a firing mechanism, including at least one trigger (11) operated by the user's hand, a grip (12) held by the user's other hand, and a selective locking mechanism (100) of said firing mechanism, operatively switchable from a deactivated position, in which it allows firing of the weapon, to an activated position, in which it prevents firing of the weapon. According to the invention, said device comprises: means to control the operating condition of said locking mechanism of said firing mechanism (13), which are provided in said grip (12) and, when the weapon is gripped correctly and naturally by the user, automatically cause deactivation of said selective locking mechanism (100), allowing firing of the weapon (A), while, when said grip (12) is released by the user, automatically cause activation of said locking mechanism of said firing mechanism, preventing firing of said weapon (A).