Andrew Smith, 34, drowned after trying to rescue his girlfriend's daughter who had fallen into the Anglezarke reservoir near Chorley last night (wed).
Police were called to the reservoir at 7.40pm after reports a man had got into difficulties in the water.
Firefighters, Mountain Rescue teams and an ambulance were scrambled after a woman told patrols her boyfriend had been struggling in the water.
A spokesman for the police said: "Patrols attended and spoke with a lady who told them that her 12-year-old daughter had fallen into the water.
"The mother's boyfriend Andrew Smith jumped in to assist the daughter but she had managed to swim to the side.
"He got into difficulties and subsequently drowned. His body was recovered at 9.45pm."
The emergency services sealed off a stretch of road as they worked to recover the body and four fire crews and Mountain Rescue teams left the scene at approximately 10.30pm.