Seat and seating system

A seat (10) comprises a front shell (11) and a back shell (12) in stacked arrangement, the front shell (11) having a seating portion (15) and a backrest (16) integrally moulded of synthetic resin, the back shell (12) having a seating support portion (19) and a backrest support portion (20) integrally moulded of synthetic resin, wherein the seating support portion (20) forms at its side facing away from the front shell (11) at least one connection interface (24) for mounting on a crossbeam (2) passing underneath the seating support portion (20) in a direction parallel to the backrest support portion (19), each connection interface (24) having at least two protrusions (25, 26) spaced apart from each other across the crossbeam (2) and configured to laterally embrace the crossbeam (2) to thereby position the seat (10) on the crossbeam (2) in transversal direction of said crossbeam (2). This enables a flexible seating system with high stability, which is simple in construction and production and can be adapted easily to various surroundings.