An edge adapter for electrical box extension rings includes a flexible adapter body enclosing and defines an adapter channel, the flexible adapter body has a continuous sidewall defining at least four sides, each side having an inner wall surface facing the adapter channel and an outer wall surface opposite to the inner wall surface. A first pair of retention flanges are adjacent one another and disposed on and radially projecting from the outer wall surface on a first side of the adapter body. A second pair of retention flanges are adjacent one another and disposed on and radially projecting from the outer wall surface on the first side of the adapter body, the first pair of retention flanges and the second pair of retention flanges together defining a first retention slot shaped and dimensioned to receive a first inner wall of an electrical box extension ring. A third pair of retention flanges are adjacent one another and disposed on and radially projecting from the outer wall surface on the second side of the adapter body opposite the first side. A fourth pair of retention flanges are adjacent one another and disposed on and radially projecting from the outer wall surface on the second side of the adapter body, the third pair of retention flanges and the fourth pair of retention flanges together defining a second retention slot shaped and dimensioned to receive a second inner wall of the electrical box extension ring.