Hernandez will spend the 2014-15 season at the Santiago Bernabeu, after a deal was finalised on Monday.
An official statement from the European champions read: "Real Madrid C. F. and Manchester United have reached an agreement for the loan of Javier Chicharito Hernandez, who will be at the club throughout this season.
"Real Madrid have the option to purchase the player.
"Javier Chicharito Hernandez will appear before the media in the Santiago Bernabeu press room today at 16:30 [15:30 BST]."
The signing of Hernandez adds further depth to a Real side bursting at the seams with attacking talent.
And while it is almost certain that the Mexico international will play a supporting role to the likes of Cristiano Ronaldo, Gareth Bale, Karim Benzema and James Rodriguez, the club's packed fixture list could give him several chances to shine.
Hernandez has made just two appearances for United this season, both coming in defeats.
He started the club's 2-1 opening-day loss to Swansea City at Old Trafford but was hauled off at half-time, and he did little to convince new United manager Louis van Gaal in last week's shock 4-0 League Cup defeat to third-tier outfit MK Dons.
The striker joined United in 2010 from Guadalajara and quickly established himself as a fans' favourite.
He scored 19 goals in 44 appearances in all competitions in 2010-11 as the Old Trafford outfit won the Premier League and reached the UEFA Champions League final.
Hernandez weighed in with 10 goals in each of the 2011-12 and 2012-13 Premier League seasons but managed just four top-flight goals last term in a campaign that saw him make only six starts.
