A bathtub closure system may include a step configured to be retrofit onto a bathtub and a plug. The step may include a first side panel, a second side panel, and an elongated platform defining a cavity configured to facilitate ingress and egress into the bathtub, and a first attachment recess extending downward from the first top surface less than an entire height of the first side panel or extending downward from the second top surface less than an entire height of the second side panel. The plug includes a body and a first attachment member affixed to the body and corresponding to the first attachment recess defined by the step. The plug cooperates with the step to form a substantially watertight seal when the first attachment member is mated with the first attachment recess and the body of the plug is positioned at least partially within the cavity.