Hydrophis brookii is a species of venomous sea snake in the subfamily Hydrophiinae of the family Elapidae. The species is native to bodies of water in Southeast Asia.
The specific name, brookii, is in honor of British adventurer James Brooke, who became the first White Rajah of Sarawak.
Males of H. brookii may attain a total length of 103.5 cm (40.7 in), which includes a tail 11.5 cm (4.5 in) long. Females are slightly smaller, and may attain a total length of 96.5 cm (38.0 in), with a tail 7.5 cm (3.0 in) long.
H. brookii is found in the Indian Ocean (Malaysia, Vietnam, western Indonesia: Sumatra, Java, Kalimantan) and the Gulf of Thailand. It is also found in a freshwater lake in Thailand.
H. brookii is viviparous.