Radamel Falcao will return to the Premier League next season after agreeing terms to join champions Chelsea on a temporary basis.
The Colombia international moved to Manchester United last September, signing a year-long loan at Old Trafford.
That agreement was with a view to a permanent move, but Falcao failed to fire at United, scoring just four goals in 26 league appearances.
Chelsea manager Jose Mourinho has previously hinted the club could make a move for the striker, and on Friday the switch was confirmed.
Falcao told Chelsea's official website: "I am very happy to be joining Chelsea and can't wait to start training and help with our aim of retaining the league title and being successful in Europe."
The 29-year-old became one of the top strikers in European football with successful spells at Porto and Atletico Madrid, winning the Copa del Rey and UEFA Europa League with the Spanish side.
That led to his move to Ligue 1 with Monaco, however, the forward struggled to settle in France and, after being unable to find his feet with United, will rook to revitalise his career at Stamford Bridge under the guidance of Mourinho.
Falcao adds to a forward line that already includes Diego Costa, who impressed significantly in his first Premier League season, and France international Loic Remy.
The former River Plate man did not find the net in four appearances as Colombia reached the quarter-finals of the Copa America before losing to Argentina on penalties.
Chelsea now boast four players who have spent time at Atletico, with Falcao joining Costa, Filipe Luis and goalkeeper Thibaut Courtois at the club.
Croatian midfielder Mario Pasalic - who joined Chelsea from Hajduk Split last year - will go the other way to spend the 2015-16 season at Monaco.
