A seat lock structure for a saddle riding vehicle, comprising: a seat (17, 57), on which a rider or a passenger sits; a key cylinder; and a seat lock mechanism to open and close the seat (17, 57) by a key operation of the key cylinder, wherein the key cylinder includes a turning arm, which is turnable by a key operation only when a key is inserted into the key cylinder, the 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 seat (17, 57) side, the second bracket integrally comprises a vertical wall, which slides with the rear wall of the first bracket, and a front extension portion, which extends forwardly from the lower portion of the vertical wall, wherein the front extension portion includes a longitudinally placed planar extension portion main body and an L-shaped slide regulating portion, which is formed to extend leftwardly and upwardly from a lower portion of a distal end of the extension portion main body, a front end portion of the extension portion main body and the L-shaped slide regulating portion constitute an engagement portion, which has a substantially U-shape in front view, in which the turning arm enters to be engaged therewith, the turning arm extends along the engagement portion, wherein the seat lock structure is configured such that the second bracket is slid in a sliding direction, when the extension portion main body is pressed by the turning arm, and such that the L-shaped slide regulating portion regulates the second bracket from being slid in the sliding direction when the key is not inserted into the key cylinder, and such that, when the key is extracted from the key cylinder after the seat is opened, a movement position of the second bracket and a turning position of the turning arm are held in the state at that time, and such that, when the seat is closed, the turning arm is unturnable unless the key is inserted into the key cylinder. a seat (17, 57), on which a rider or a passenger sits; a key cylinder; and a seat lock mechanism to open and close the seat (17, 57) by a key operation of the key cylinder, wherein the key cylinder includes a turning arm, which is turnable by a key operation only when a key is inserted into the key cylinder, the 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 seat (17, 57) side, the second bracket integrally comprises a vertical wall, which slides with the rear wall of the first bracket, and a front extension portion, which extends forwardly from the lower portion of the vertical wall, wherein the front extension portion includes a longitudinally placed planar extension portion main body and an L-shaped slide regulating portion, which is formed to extend leftwardly and upwardly from a lower portion of a distal end of the extension portion main body, a front end portion of the extension portion main body and the L-shaped slide regulating portion constitute an engagement portion, which has a substantially U-shape in front view, in which the turning arm enters to be engaged therewith, the turning arm extends along the engagement portion, wherein the seat lock structure is configured such that the second bracket is slid in a sliding direction, when the extension portion main body is pressed by the turning arm, and such that the L-shaped slide regulating portion regulates the second bracket from being slid in the sliding direction when the key is not inserted into the key cylinder, and such that, when the key is extracted from the key cylinder after the seat is opened, a movement position of the second bracket and a turning position of the turning arm are held in the state at that time, and such that, when the seat is closed, the turning arm is unturnable unless the key is inserted into the key cylinder.