P Warp Door

P Doors are Warp Doors that can only be entered when a P Switch is active. These Warp Doors made their first appearance in Super Mario Bros. 3, but with a different appearance from its current one.

Super Mario Bros. 3
A P Door first appears in Super Mario Bros. 3 in the second fortress of Giant Land. This P Door is invisible, and an outline of Blue Coins will show where to enter when the P Switch is pressed. P Doors also appeared in later fortresses including World 7-Fortress 1 and World 8-Fortress, this time in visible form, represented as a white rectangular doorway.

Super Mario World
In Super Mario World, P Doors appear in the levels Donut Secret House, Vanilla Ghost House and Forest Ghost House. They have the same sprite as a Warp Door, but with a light blue palette.

Super Mario Maker / Super Mario Maker for Nintendo 3DS
In Super Mario Maker, P Doors were introduced in the version 1.30 update. They have new sprites for all styles, in the same blue as the P Switches with a white or blue "P". Unlike in Super Mario World, they are visible as outlines when not active.

Trivia

 * In the Ghost House of World 3 in New Super Mario Bros., there are doors that only appear during the effect of a pressed "?" Switch, thus functioning in a similar way to P Doors.