To provide a seat with a backrest for a boat capable of being easily attached/detached to a stool panel of the boat, preventing hip numbness, pain and backache, and being compactly stored.

U-shaped hook parts 1d are provided at front ends of a cushion frames and hooked to the boat stool panel 10. A back frame 2g provided with friction force is rotatably mounted to a cushion hinge part 1e. When the back frame is erected, a back hinge part 2i and the hook part 1d hold the boat stool panel 10 between them to tighten and fix the seat. A cushion board 1c is mounted to the cushion frames and a back board 2h is mounted to the back frame for connecting the left/right cushion frames and holding a pad trim assembly 9. A hook-and-loop fastener 6 is mounted to the back board 2h, the cushion board 1c, and the pad trim assembly 9 for easily attaching/detaching the pad trim assembly 9 to the frames.