A synthetic resin-made thrust sliding bearing 1 includes a synthetic resin-made upper casing 12, a synthetic resin-made lower casing 22, a synthetic resin-made thrust sliding bearing piece 33 interposed between the upper casing 12 and the lower casing 22, an outer elastic seal member 47 having an inner elastically deformable lip portion 43 and an outer elastically deformable lip portion 46, and an inner elastic seal member 54 having an outer elastically deformable lip portion 50 and an inner elastically deformable lip portion 53.