Abstract: A support brace having two complementary sections that are matingly received in the opposite sides of the opening of a window frame to be installed in a poured concrete wall. The two complementary brace sections are secured together to form a bracing structure that is matingly received in the interior opening of a window frame. The width of the support brace is adjustable at a number of selected widths to accommodate window frames of selected widths. Both the inside section of the brace and the outside section of the brace have slotted plates that overlap each other so they can be pinned together to hold the two sections of the support brace at a predetermined and adjustable distance apart corresponding to the width of the window frame being installed. The support brace has both tie slots bars for receiving concrete form ties used with metal forms, and fastener brackets for attachments to wooden forms by nails and screws.