Eriastrum sapphirinum is a species of flowering plant in the phlox family known by the common name sapphire woollystar.

It is an annual reaching anywhere from 5 to 40 centimeters in height, forming clumps or singular spindly stems.

The stem is erect and may be reddish to green, and has the occasional threadlike leaf with sparse hairs to a coat of dense wool.

The inflorescences at the tips of the stems are packed with pointed, leaflike green to red bracts and funnel-shaped flowers.

The corolla of the flower has five lobes each one half to one centimeter long and pale to bright blue.