Mawson's future has been the subject of intense transfer speculation in recent weeks after the Swans were relegated from the Premier League last season and joins the Cottagers as they continue to make impressive moves in the transfer market.
The 24-year-old said, as quoted by Fulham's website:
“I’m excited to be a part of this great club moving forward and to help achieve success in the Premier League."
Fulham chairman Tony Khan added:
“Alfie is also very highly regarded by our scouting department and our data analysts, giving us great confidence in our back line as we prepare long term for Premier League competition.
“It’s only fitting that such a fine homegrown talent like Alfie will flourish in the presence of the finest supporters in London at Craven Cottage for years to come. Come on Fulham!”