An adapter for mounting electric units to be wired to an electric system, in particular for mounting contact units and/or LED units adapted to be connected to one or more electrical systems by wiring with one or more wires to be inserted into respective terminals to a support, such as a DIN or miniDIN rail having longitudinal opposite edges, which adapter comprises a substantially box-like and internally hollow anchoring body having: - an upper portion with an upper face provided with a hole for the passage of a control for driving one or more electric units; - a lower portion removably coupled with said upper portion and having a bottom wall; - a pair of reciprocally opposite side faces at least partially open to allow the wires to access to at least one electric unit housed into said anchoring body; - means for removably mounting said anchoring body to the support; - first holding means adapted to removably fix at least one electric unit to said bottom wall, said at least one electric unit having a case housing at its inside switching means and being provided with holding fins for coupling with the lower face of said case; - a second holding means for removably fixing at least one another electric unit on said upper face, said at least one another electric unit having a case which houses at its inside switching means and being provided with holding fins for coupling with the upper face of said case; - wherein said lower portion and said upper portion comprise respective first coupling means (18, 19) of the male and female type for their mutual and removable coupling; wherein said first holding means arc firmly associated with said bottom wall and comprise at least one first and one second recesses formed in said bottom wall at respective of said opened side faces and adapted to house and snap-hold corresponding holding fins of an electric unit to fix it to said lower portion also in absence of said upper portion; wherein said second holding means are removably inserted in said anchoring body to be used selectively and alternatively to said first holding means and are adapted to house and snap-hold the corresponding holding fins of the respective electric unit to fix it to said upper portion in absence of said lower portion; characterized in that said mounting means are associated with said lower portion and are of the snap-fit type with at least one pair of grooves extending along said bottom wall, inferiorly thereto and parallel to respective open side faces of said anchoring body, for snap fitting respective longitudinal edges of the support, and in that said opposite side faces have respective openings which extend both in said lower portion and in said upper portion and fill up substantially their whole extension for allowing direct access to all the corresponding of said recesses to simplify the access to the terminals of the electric units. - an upper portion with an upper face provided with a hole for the passage of a control for driving one or more electric units; - a lower portion removably coupled with said upper portion and having a bottom wall; - a pair of reciprocally opposite side faces at least partially open to allow the wires to access to at least one electric unit housed into said anchoring body; - means for removably mounting said anchoring body to the support; - first holding means adapted to removably fix at least one electric unit to said bottom wall, said at least one electric unit having a case housing at its inside switching means and being provided with holding fins for coupling with the lower face of said case; - a second holding means for removably fixing at least one another electric unit on said upper face, said at least one another electric unit having a case which houses at its inside switching means and being provided with holding fins for coupling with the upper face of said case; - wherein said lower portion and said upper portion comprise respective first coupling means (18, 19) of the male and female type for their mutual and removable coupling;