Gallagher was one of Mauricio Pochettino's standout performers last season but decided to pursue a move away from Stamford Bridge after being made aware that he would not be a guaranteed starter under new boss Enzo Maresca.
The England international is now in the final 12 months of his current deal with the Blues after failing to agree new terms with the club.
The 24-year-old midfielder had agreed to personal terms with the La Liga outfit weeks ago, but the deal stalled after Chelsea halted their pursuit of Atletico forward Samu Omorodion.
Gallagher had been waiting in Madrid for the move to be finalised before returning to the UK, as uncertainty began to surround the deal, but it seems that he is now just hours away from completing his switch from London to the Spanish capital.
Gallagher set to be unveiled as Atletico Madrid's newest addition
Transfer expert Fabrizio Romano revealed on Tuesday that Gallagher will be presented as Atletico Madrid's newest addition on Wednesday at the Metropolitano Stadium.
"He's already signed his contract valid until 2029, also done all medical tests so just formally being announced in next 24h," Romano confirmed via a post on X.
It is understood that the clubs settled on a fee in the region of £35.8m and have included an undisclosed sell-on fee in the terms of the agreement.
Gallagher has made 95 appearances across all competitions for the Blues since joining the club's youth ranks at the age of just eight.
The 24-year-old also wore the captain's armband for the majority of last season in the absence of Reece James, who was ruled out of action through injury.
If Gallagher's move to Atletico is completed without any hiccups, he will become only the second England international ever to play for Los Colchoneros after Kieran Trippier, who helped the side win the La Liga title in the 2020-21 season.
Chelsea summer spending spree continues
It has been another busy and expensive summer at Stamford Bridge, with the Blues having already spent well over £160m on bolstering their squad under Maresca.
Chelsea have already brought in the likes of Pedro Neto for £54m from Wolverhampton Wanderers and splashed out a combined £60m for Kiernan Dewsbury-Hall and Estevao Willian from Leicester City and Palmeiras respectively.
However, it seems that the Blues are not done with their summer transfer business as yet, with Romano revealing that the club have now secured the signing of Joao Felix.
It is understood that the West Londoners have forked over around £46.3m for the Portugal international, who spent half a season on loan at Stamford Bridge during 2022-23.
Reports suggest that Felix has already completed his medical and is set to be announced as the club's newest signing imminently.