Donnie McKinnon and Peter Jackson were at a bar in Market Road, Soufriere, when the incident happened on Saturday afternoon, according to local reports.
Mr McKinnon was believed to have been living on the island for 13 years as manager of the Soufriere Estate while Mr Jackson was taken to hospital.
Freelance photographer Mr Jackson, from Lancashire, is understood to have lived on the island for three decades and remains in a critical condition in hospital.
The St Lucia Times reported that both men had lived on the island for some time, with one local telling the paper: “Everyone knew them. They were community people.
A Foreign Office spokeswoman said: “We are supporting the family of a British man who has died and a British man who has been injured in St Lucia.”