A windowsill (also written window sill or window-sill, and less frequently in British English, cill) is the horizontal structure or surface at the bottom of a window. Window sills serve to structurally support and hold the window in place.

The exterior portion of a window sill provides a mechanism for shedding rainwater away from the wall at the window opening. Therefore, window sills are usually inclined slightly downward away from the window and wall, and often extend past the exterior face of the wall, so the water will drip off rather than run down the wall.

Some windowsills are made of natural stone, cast stone, concrete, tile, or other non-porous materials to further increase their water resistance.

Windows may not have a structural sill or the sill may not be sufficiently weather resistant. In these cases, a strip of waterproof and weather resistant material (steel, vinyl, PVC) called a sill pan may be used to protect the wall and shed the water. Like the sill, a sill pan will usually be inclined and protrude from the wall.