Waterproof Soap is a magical soap that appears in the Waterproof Soap Arc of the Ranma 1/2 manga and its anime adaptation. It is apparently part of a variety of goods sold through a mail order catalog that is printed by the people who are connected to Jusenkyo.
When applied to the skin, waterproof soap temporarily makes the user impervious to getting wet. Most prominently, this means users who have been cursed by Jusenkyo will be effectively locked into the form in which they used the soap; a person who used it whilst in their normal form would find themselves not transforming when touched with cold water. The soap does eventually wear off, and a Jusenkyo cursed user can find their curse activating if it does so whilst they are already wet, but a single application can last for multiple hours and undergo repeated heavy drenchings before finally wearing off.
Shampoo purchases a bar of waterproof soap during the titular arc, hoping that it will cure her Maoniichuan curse and thus she will no longer have that obstacle to the heart of Ranma Saotome to deal with. Unfortunately, she accidentally loses it at the public bathhouse, and it is used and taken away by Ryoga Hibiki. The angry Shampoo confides in Ranma Saotome about the soap's existence and the two work together to try and steal it back from him, leading to a series of hijinks as Akane Tendo is advertently roped into the mess and Ranma continually tries to betray Shampoo to steal the soap for himself. In the end, it is revealed that the waterproof soap is not a permanent Jusenkyo cure, and Shampoo angrily derides it as a "rip-off", vowing not to trust the Jusenkyo catalog again.
- Precisely why Shampoo never tried to simply ask Ryoga for her soap back is unclear. It's possible she suspected he would refuse due to his P-chan issues, which is justified considering that Ryoga immediately declares he will never return the soap upon learning of it being the secret behind his apparent cure.
- Despite Shampoo's protests at the story's end, the waterproof soap is actually a surprisingly viable "workaround" to the life problems of being Jusenkyo cursed, considering how long in remains effective for. The only major issue is the potential cost of buying a stockpile of soap bars, as the price of a single bar is never revealed.