Cirsium hydrophilum is a species of thistle which is endemic to California, where it is found only in the San Francisco Bay Area and the Sacramento-San Joaquin River Delta.

[2] Cirsium hydrophilum may reach 2 metres (6.6 ft) in height with a branching, cobwebby stem.

The leaves are longest near the base of the plant, approaching 90 centimetres (35 in) in length.

The head is lined with sticky, twisted, spiny phyllaries and contains pink to purple flowers.

The fruit is an achene a 2–4 millimetres (0.079–0.157 in) long topped with a pappus of about 1.5 centimetres (0.59 in) centimeters.