The Spanish wing-back saw his contract at Stamford Bridge ended by mutual consent and he is now expected to join Barcelona imminently, after Pierre-Emerick Aubameyang moved in the opposite direction for a fee of £12million late on transfer deadline day.
Alonso has pushed for a switch to Camp Nou all summer and will sign a three-year contract back in LaLiga, following ex-Arsenal defender and La Masia graduate Hector Bellerin in heading to Barca.
On a busy deadline day for Barcelona, American right-back Sergino Dest was also loaned to AC Milan, while Martin Braithwaite had his contract cancelled my mutual consent ahead of a switch to local rivals Espanyol.
Chelsea, meanwhile, granted a raft of exits and also loaned Swiss midfielder Denis Zakaria from Juventus.
Alonso spent six fruitful years at Chelsea after a £23m move from Serie A side Fiorentina in the summer of 2016, thriving back in the Premier League after previous stints at Sunderland and Bolton.
Marcos Alonso has left Chelsea by mutual consent, ending a six-year association with the club.
Everyone would like to thank Marcos for his service to the Blues and wish him all the very best for the rest of his career. 💙
— Chelsea FC (@ChelseaFC) September 2, 2022
The ex-Real Madrid defender made 212 appearances across all competitions, winning a host of major silverware including the Premier League, Champions League, FA Cup, Europa League, Super Cup and Club World Cup as a dependable and popular figure who was a key performer for several managers in west London.
In a statement bidding farewell to Alonso, Chelsea wrote: “A thoughtful and extremely likeable man off the pitch, Alonso was the epitome of suave around Cobham, but it is his impact in the final third that will linger longest in the mind when his six thoroughly successful seasons in SW6 are reflected upon in the years to come.
“Everyone at Chelsea Football Club would like to thank Marcos for his service to the Blues and wish him all the very best for the rest of his career.”
Chelsea prepared for Alonso’s expected exit by signing fellow Spaniard Marc Cucurella from Brighton in a £63m transfer last month, with England’s Ben Chilwell now fit and available again after a long-term knee injury.