Chilwell has put the dotted lines on a five-year contract at Stamford Bridge as Frank Lampard works on his porous defensive department ahead of the 2020/2021 season.
Chelsea conceded 54 goals in the Premier League last season, which was the most among the top ten sides.
Chilwell came through the youth ranks at Leicester City and made 129 appearances for the Foxes, scoring four goals.
"I'm delighted to be joining Chelsea at this very exciting time for the club," Chilwell said.
"I'm looking forward to being part of this young, dynamic squad led by Frank Lampard as we challenge for honours next season.
"I can't wait to get started and hopefully it won't be long before we're playing in front of the Chelsea fans at Stamford Bridge."
Chelsea director Marina Granovskaia added: 'We are very pleased to have completed our third addition to our exciting squad for the coming season.
'Ben brings plenty of top-level experience playing in the Premier League, Champions League and at international level, despite his young age.
"We are confident Ben will adapt very quickly to being a Chelsea player and with the fixtures programme for 2020/21 especially busy, he is a fantastic addition to the squad needed to challenge for honours."
Chilwell joins Hakim Ziyech and Timo Werner in moving to Chelsea for the upcoming season.
Former Paris Saint Germain defender Thiago Silva is expected to join the Blues on a free transfer in the coming days.
