On deadline day, the 31-year-old agreed to leave Chelsea with one year remaining on his contract.
Alonso, who hasn’t played for the Blues this year, has now decided to sign a one-year contract with Barca that contains a release clause of 50 million euros (£43.3 million).
He assisted Chelsea in winning the Premier League and the Champions League after moving from Fiorentina to Chelsea in 2016 and scored 29 goals in 212 games.
Alonso, whose father was a 1982–1987 member of the Barcelona team, went through the Real Madrid youth system before joining Bolton in 2010 and Fiorentina in 2013.
He is the third defender to leave the Premier League this summer to join the Spanish powerhouses on a free transfer.
On the last day of the transfer window, Spain’s Hector Bellerin re-joined Barcelona from Arsenal, while Denmark’s Andreas Christensen, a center defender, also signed after leaving Chelsea.