A boat propulsion system includes a shift mechanism arranged to change a shift position among forward, neutral, and reverse. A shift position detection mechanism is connected to a control lever with a wire. The shift position detection mechanism has a detected member and a position detection section. The detected member is displaced corresponding to an operation position of the control lever as the control lever is operated. The position detection section detects a position of the detected member. The position detection section outputs a shift position signal corresponding to the detected position of the detected member. A control device controls the shift position of the shift mechanism on the basis of the shift position signal. The boat propulsion system provided with the shift mechanism of an electronic control type can replace a boat propulsion system provided with a shift mechanism of a mechanical type at low cost.