A thrust sliding bearing 1 includes: a synthetic resin-made upper casing 2; a synthetic resin-made lower casing 3 which is superposed on the upper casing 2 so as to be rotatable about an axis O in a circumferential direction R relative to the upper casing 2; a synthetic resin-made thrust sliding bearing piece 5 disposed in an annular space 4 between the upper casing 2 and the lower casing 3; and a synthetic resin-made seal member 8 for sealing respective other end portions, communicating with the outside, of a gap 6 on an inner peripheral side and a gap 7 on an outer peripheral side in a radial direction A between the upper casing 2 and the lower casing 3, the gap 6 and the gap 7 respectively communicating with the annular space 4 at their one annular end portions.