Bosses at Preston Council said the move was as a mark of respect, and in solidarity with the people of Orlando and the lesbian, gay, bisexual and transgender community.
Coun Robert Boswell, cabinet member for environment, said: “It’s such an appalling tragedy and our hearts go out to the victims and their families of this shocking event.
“It may have taken place far away but we are all still touched by the human emotion and outpouring of grief when so many people are killed and injured.
“We are flying the rainbow flag at half mast as a mark of respect and in solidarity with the people of Orlando as well as the lesbian, gay, bi-sexual and trans-gender community who have been profoundly affected by this tragedy.”
The massacre saw 50 people gunned down at a night out at Pulse, a well-known gay venue, at about 2am on Sunday.
It is the deadliest mass shooting in US history.
The flag will be put up today at the Town Hall in Lancaster Road.