The 63-year-old Portuguese is set to replace Alvaro Arbeloa, who took over from new Chelsea boss Xabi Alonso in January and has steered the club to a guaranteed second-placed LaLiga finish, although they sit a gaping 11 points behind champions Barcelona with one game remaining.
For their part Barcelona have reached a deal to extend their German coach's contract until 2028 with the option for an another season.
The club said the 61-year-old Flick, whom it calls the "architect of a winning Barça," signed his new contract on Monday.
Flick, who has led Barca to successive titles, has said he doesn't plan on coaching elsewhere after Barcelona. He arrived in May 2024 to replace Xavi Hernández having previously coached Bayern Munich and Germany's national team.
Mourinho spent three years in charge at the Santiago Bernabeu between 2010 and 2013, guiding them to a title as the first La Liga team to amass 100 points, and to the Copa del Rey.
His impending return follows weeks of speculation that he was the man president Florentino Perez wanted to take Real forward as they attempt to challenge Barca, winner of three of the past four La Liga titles.
Perez has come under increasing pressure in recent weeks after two seasons without a trophy for Real, and faced protests during last Thursday evening's 2-0 league win over Oviedo at the Bernabeu.
Mourinho's return marks a significant comeback for one of the biggest figures in world football.
He has inhabited the brightest spotlight in the European game, managing clubs including Manchester United, Inter Milan and Chelsea.
But his time at the game's highest levels had appeared over when he took a job away from Europe's big five leagues to manage Turkiye's Fenerbache in 2024.
Mourinho was appointed Benfica boss on a two-year contract in September last year and has since led the Portuguese club to a third-placed Primeira Liga finish behind Porto and Sporting Lisbon, during which they went unbeaten.
However, Mourinho is understood to have a modest release clause in his deal which has allowed Real to prise him free relatively inexpensively.
Repeatedly touted as a future Portugal manager, Mourinho remains one of world football's larger than life figures.
He began to learn his trade under Sir Bobby Robson, initially serving as the Englishman's interpreter during his time in charge at Sporting and Porto before following him to Barca and working as his assistant.
After launching his managerial career in Portugal, he led Porto to two Primeira Liga successes and Champions League glory in 2004.
He won another European crown with Inter as well league titles in England, Spain and Italy among a glittering array of domestic and continental trophies.
12°C