Abstract: A rotary blade blocker for preventing inadvertent harmful firing of the engine of a rotary-type mower during rotary blade removal and reinstallation or during removal of mowing debris from within the mower deck. The blade blocker includes a clamping portion temporarily clampingly securable to a lower edge of the deck and a blocking portion rigidly supported by the clamping portion and positioned within the deck perimeter to make contact with the distal portion of the rotary blade to facilitate safe blade changes and during manual debris removal from within the deck when the blade blocker is temporarily secured to the edge of the deck.