A wave of muddy water swept through the town of Sant Llorenc des Cardassar after a river burst its banks following heavy rain on Tuesday evening.
The Britons killed were travelling in a taxi which was overcome by floodwater, The Telegraph reports.
The Foreign Office said in a statement: “We are in contact with the Spanish authorities responding to flooding in Majorca, and urgently seeking updates from them.”
Videos posted to social media show water flooding through towns and cars being swept away.
At least another 12 people were still missing after the downpours, local media reported.
Antonia Bauza, the deputy mayor of Sant Llorenc, confirmed two of the victims were British nationals.
She told local radio that two other victims were local, including an elderly woman whose body was found in the basement of her house.
The regional government called an emergency meeting to coordinate rescue efforts and authorities said rescue workers and military units were heading to the area to help.
“My solidarity and support goes out to the families and friends of victims and all those affected by these tragic floods,” Spanish Prime Minister Pedro Sanchez tweeted early on Wednesday.
Hundreds of displaced residents are seeking shelter in emergency relief centres, while soldiers have been sent to assist the rescue effort, Spanish authorities said.
The Spanish weather agency reported around 8in (20.3cm) of rain fell in four hours on Tuesday evening in parts of the island.