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