Mayoral, 23, joins the club on an initial two-year loan deal, for a fixed fee of €2 million.
As part of the agreement, Roma has the option to make the transfer permanent at any point during the 2020-21 season for a fee of €15m.
In the second season, the cost to exercise this option rises to €20m.
Should either option be exercised, the player has already agreed a contract with the club which will run until 30 June 2025.
“I am pleased to finally join – it has been a hectic few days, but the main thing is everything has worked out for the best,” Mayoral said.
“It’s an honour for me to represent an historic club like Roma. I am grateful to the club for the faith they have shown; I will give everything for this club and I hope to make our fans proud.”
Mayoral, a Spain Under-21 international, has also represented his country at Under-17 and Under-19 level.
He has made 33 appearances for Real in his career, scoring seven goals.
A product of the Real Madrid youth academy, he has previously enjoyed loan spells at VfL Wolfsburg and Levante, prior to joining the Giallorossi.
“With this signing we are adding a talented player to the group, and one who has learned his trade at a renowned club like Real Madrid,” said Guido Fienga, the club’s CEO.
“Despite his young age he has already picked up some valuable experience. He had the choice to stay at Real Madrid or come here – he chose Roma, and we are delighted about that.”
Mayoral will wear the No. 21 shirt.
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