There is provided a seat lock structure for a saddle riding vehicle which is user-friendly and can be downsized. A key cylinder includes a turning arm which is turned by a key operation, a seat lock mechanism includes a first bracket which is fixed to a vehicle body frame, and a second bracket which is slidably supported on the first bracket and engageable with a rear seat side, and the second bracket is slid by being pressed by the turning arm, and an L-shaped end portion formed on the second bracket regulates the second bracket from being slid when the turning arm is unturnable.