Reinforced concrete wide beam flooring systems are used increasingly in seismic regions, despite little information being available on their performance under seismic actions. This paper presents the results of a set of experiments performed on two full-scale exterior wide beam–column connections, which were tested under lateral quasi-static reversed cyclic loading. The control specimen had both longitudinal and transverse reinforcement in its spandrel beam. The second specimen was built in the same way but provided with post-tensioning force. The specimens were tested under a combination of axial force and quasi-static reversed cyclic lateral displacements. Results show that the control specimen reached its expected beam flexural capacity at a drift ratio of 3%, and then the joint shear failure and spandrel beam torsional failure controlled the behaviour. The seismic performance of the specimen with a post-tensioned spandrel beam was considerably improved compared to the control specimens. The specimen sustained a 5% drift without any significant drop in strength. No signs of joint shear failure or spandrel beam torsional failure were observed in this specimen. The results indicate that adopting a post-tensioned spandrel beam can not only prevent torsional failure, but can also improve the joint shear capacity and displacement ductility.