The centre-back completed his switch from Swansea on Thursday in a deal believed to be worth £15m.
"I'm really happy to get this over the line and join Fulham," the centre-back told the club website.
"I'm excited by the project going on here and the football being played last season was lovely on the eye but also effective and I can't wait to be a part of it and get going.
"I'm excited to be a part of this great club moving forward and to help achieve success in the Premier League."
Welcome to @fulhamfc, @AlfieMawson!
— Fulham Football Club (@FulhamFC) August 2, 2018