A belt buckle (10) for a seat belt includes a buckle housing (16), a locking mechanism for locking a plug-in tongue inserted in the buckle housing (16), a release button (22) which is movable in a first direction relative to the buckle housing (16) for unlocking the plug-in tongue, and a safety mechanism for preventing automatic displacement of the release button (22) in the first direction. The safety mechanism includes a mass body (36) movable in the first direction relative to the buckle housing (16) and a movably supported coupling lever (24) including an axis of rotation which is stationary relative to the buckle housing (16). The coupling lever (24) is coupled to the mass body (36) and to the release button (22) so that, upon acceleration of the belt buckle (10) in a second direction opposed to the first direction, it transmits an inertia force acting on the mass body (36) in the first direction to the release button (22) in the opposite second direction.