A pump action firearm with bolt slide locking mechanism includes a receiver, a barrel assembly coupled by the receiver, and a bolt slide slideably disposed in the receiver for movement between rearward open breech and forward closed breech positions. A slide lock pivotably mounted to the bolt slide about a pivot axis is selectively engageable with an interference surface on the receiver. When the firearm is fired via a trigger pull, a rotatable hammer strikes and actuates the slide lock. The slide lock rotates from a locked position engaged with the interference surface to prevent the slide from moving out of the closed breech position, to an unlocked position disengaging the interference surface. This allows the slide to be manually moved to the open breech position for cycling the action. A manual actuator is provided for unlocking the slide lock absent a trigger pull.