Salix udensis (syn. S. sachalinensis F.Schmidt) is a species of willow native to northeastern Asia, in eastern Siberia (including Kamchatka), northeastern China, and northern Japan.

It is a deciduous shrub growing to 5 m (16 ft) tall. The leaves are slender, lanceolate, 6–10 cm long and 0.8–2 cm broad, glossy dark green above, glaucous and slightly hairy below, with a serrated margin. The flowers are produced in early spring on catkins 2–3 cm long.

The cultivar S. udensis 'Sekka' (Japanese fantail willow) is grown as an ornamental plant; it has fasciated stems (stems that are joined abnormally in a flattened arrangement--hence "fantail"), highly prized by Ikebana flower arrangers.