Coupling unit for panels, which comprises: - a base body intended to be fixed to a support structure; wherein said base body is provided with a bottom face intended to be abutted against said support structure and with an opposite front face, and is provided with two opposite sides, at each of which a lateral side of at least one corresponding panel is intended to be arranged, in abutment against said front face, such lateral side being provided with a longitudinal groove; - at least two retention bodies, which are rigidly fixed to said base body and each comprise a stem portion which is extended, along an extension direction (Y') substantially orthogonal to the front face of said base body, between a first end, which is directed towards the front face of said base body, and an opposite second end; wherein said stem portion is provided with at least one through hole, extended from said second end to said first end, and is provided with a first face and with a second face which are directed in opposite directions with respect to each other and are extended between said first end and second end; wherein said first face is oriented only on the corresponding said side of said base body in order to be directed towards the lateral side of said panel arranged on said side; wherein each said retention body is provided, only on said first face of said faces (17, 18), with an engagement portion, which is projectingly extended from said first face and is positioned only on the corresponding said side of said base body for being inserted in the longitudinal groove of the lateral side of said panel; wherein said at least two retention bodies are positioned with their respective engagement portions arranged, with respect to each other, on opposite said sides of said base body; wherein said coupling unit comprises, for each said retention body, at least one adjustment element, which is inserted in said through hole, is engaged with said retention body and is configured for rigidly fixing said retention body to said base body in an adjustable manner along said extension direction (Y'); wherein said base body is provided at least with a guide extended parallel to said extension direction (Y') and with which said retention body is slidably engaged; characterized in that said guide comprises at least one guide portion projectingly extended from the front face of said base body, and said retention body is provided with at least one guide seat in which said guide portion is slidably inserted. - a base body intended to be fixed to a support structure; wherein said base body is provided with a bottom face intended to be abutted against said support structure and with an opposite front face, and is provided with two opposite sides, at each of which a lateral side of at least one corresponding panel is intended to be arranged, in abutment against said front face, such lateral side being provided with a longitudinal groove; - at least two retention bodies, which are rigidly fixed to said base body and each comprise a stem portion which is extended, along an extension direction (Y') substantially orthogonal to the front face of said base body, between a first end, which is directed towards the front face of said base body, and an opposite second end; wherein said stem portion is provided with at least one through hole, extended from said second end to said first end, and is provided with a first face and with a second face which are directed in opposite directions with respect to each other and are extended between said first end and second end; wherein said first face is oriented only on the corresponding said side of said base body in order to be directed towards the lateral side of said panel arranged on said side; wherein each said retention body is provided, only on said first face of said faces (17, 18), with an engagement portion, which is projectingly extended from said first face and is positioned only on the corresponding said side of said base body for being inserted in the longitudinal groove of the lateral side of said panel; wherein said at least two retention bodies are positioned with their respective engagement portions arranged, with respect to each other, on opposite said sides of said base body; wherein said coupling unit comprises, for each said retention body, at least one adjustment element, which is inserted in said through hole, is engaged with said retention body and is configured for rigidly fixing said retention body to said base body in an adjustable manner along said extension direction (Y'); wherein said base body is provided at least with a guide extended parallel to said extension direction (Y') and with which said retention body is slidably engaged; characterized in that said guide comprises at least one guide portion projectingly extended from the front face of said base body, and said retention body is provided with at least one guide seat in which said guide portion is slidably inserted.