SAI or Easy Paint Tool SAI (ペイントツールSAI) is a lightweight raster graphics editor and painting software for Microsoft Windows developed by Koji Komatsu (小松 浩司, Komatsu Kōji) and published by Systemax.

Various raster drawing tools are implemented, such as the Airbrush, Watercolor, Pen, and Marker, which can all be easily customized, and stored in slots in the user interface of the application.

Any series of transforms can be set up and then applied at once to a specific selection minimizing the softening of the image.

Users may use another program for more complex editing, but when the image is brought back to SAI, its properties may be changed.

Existing brush presets can be edited, and the user has the choice of adding custom ones by placing bitmap files into the "elemap" folder.

New canvas presets, as well as custom brush textures may be added by the user, in the form of grayscale bitmaps.

In parallel with the updates to SAI, development began on version 2, in which the software is being rebuilt from the ground up.