Steel blue is a shade of blue that resembles blue steel, i.e., steel which has been subjected to bluing in order to protect it from rust. It is one of the less vibrant shades of blue; it is usually identified as a blue-grey color.

The first recorded use of steel blue as a color name in English was in 1817.

In 1987, Steel blue was included as one of the X11 colors when that color list was formulated. After the invention of the world wide web in 1991, these colors became known as the X11 web colors.