A structure of a vehicle side door includes: an inner panel including a first protrusion and a second protrusion alternately provided in a vehicle front-rear direction at an upper end of the inner pane, the first protrusion protruding to the inner side, the second protrusion protruding to an outer side; an inner reinforcement that has a flange and is provided along the vehicle front-rear direction, the flange including a joint and a cutout alternately provided in the vehicle front-rear direction, the joint being joined to a surface of the first protrusion on the outer side, the cutout exposing the second protrusion from the flange to the outer side; and a weatherstrip provided along the vehicle front-rear direction, the weatherstrip including a pinching part that contacts a surface of the first protrusion on the inner side and a surface of the second protrusion on the outer side.