Horse Under Water (1963) is the second of several Len Deighton spy novels featuring an unnamed British intelligence officer.

The novel is set mostly in a small fishing village in Portugal, during the António de Oliveira Salazar dictatorship.

It retains the style of The IPCRESS File — multiple plots twists, Gauloises cigarettes, the grime- and soot-stained British winter.

The plot centres on retrieving items from a Type XXI U-boat sunk off the Portuguese coast in the last days of World War II.

A 1968 film adaptation of Horse Under Water was planned, but following the poor reception of Billion Dollar Brain it was abandoned.