A security door brace is provided for bearing against an inside surface of a barrier door in order to prevent non-permitted, inward opening of the door. The security door brace carries one or more elongated pin means for cooperative engagement with one or more slotted receiving means formed within a cooperating mounting plate that is firmly affixed, preferably via conventional screw fastener means, to a portion of the floor underlying the intended operable position of the security door brace.