A sear mechanism for firearms comprises a sear box 12, and an additional safety device to prevent the hammer 16 from releasing accidentally and to fire under the action of inertial forces acting along the direction of the axis of the barrel in case of impact of the firearm, the additional safety device being an interposing element 60 which is slidably mounted below the firing pin latch 43 in a housing having a first inclined plane element 63 and a second inclined plane element 63a which are arranged between lateral walls 62 to form a seat 61 which holds element 60 in a first position where it does not interfere with the operation of the firing pin latch 43 but allows said element 60 to move into a position where it prevents the pin latch 43 from moving downwardly to disengage hammer 16.