A water jet propulsion watercraft can be provided with: a supporting member that supports a straddle-type seat on a deck thereof. Steering handlebars can be disposed in front of the seat. A space can be formed between the seat and the deck directly underneath the seat, extending approximately over the entire length of the seat in a fore-and-aft direction of the watercraft. The supporting member can have an upper portion secured to the seat, a lower portion secured to the deck, and an intermediate portion connecting the upper portion and the lower portion. The intermediate portion can be provided at either the front end or the rear end of the seat.