Lupinus hirsutissimus is an erect annual herb growing to one meter tall; it may exceed one meter in habitat recovering from wildfire. The stem and herbage are coated in long, stiff hairs that sting skin when touched. Each palmate leaf is made up of 5 to 8 leaflets up to long and 1 or 2 wide. The inflorescence bears several flowers generally not arranged in whorls. Each flower is between 1 and 2 centimeters long and dark pink in color with a yellowish to pinkish spot on its banner. The fruit is a hairy legume pod up to long.
