Hydrophis semperi, (or Philippine freshwater sea snake, Garman's sea snake or lake snake) is a rare species of venomous sea snake found only in a single lake on the island of Luzon in the Philippines. It is noted for being one of two known species of sea snake (and the only one in its family) that is known to be found almost-exclusively in freshwater. The other freshwater sea snake species is Laticauda crockeri.