Erysimum diffusum is a species of flowering plant in the family Brassicaceae, native to western North America. It is a perennial herb growing from a woody base and producing a slender, erect stem up to about 50 cm tall. The leaves are linear to lance-shaped and up to 10 cm long. The inflorescence is a raceme of bright yellow flowers with four petals each up to 2 cm long.