The 34-year-old has joined La Liga side Elche.
By the end of his loan, Casilla's Elland Road contract will have 12 months to run.
Casilla's status as first-choice goalkeeper was effectively ended by being found guilty of using racist behaviour in 2020, even though Leeds stood by him at the time.
The Spaniard's eight-match suspension opened the door to youngster Illan Meslier whose performances since have established him as the No 1 at the club.
Casilla joined from Real Madrid midway through the 2018-19 and although he ended it on a low, making a bad decision in the play-off semi-final second leg against Derby County, he was part of a team which had an outstanding defensive record in the first half of the following season.
But he struggled during a difficult period of results for the team and was already under pressure from outside the club when he was found guilty of directing racial abuse at Charlton Athletic's Jonathan Leko in an October 2019 game.
Leeds were very unhappy about the ban, based on the balance of probability with no decisive proof either way.
With Meslier excelling during the season's run-in, Leeds nevertheless retained Casilla as their back-up goalkeeper, and as well as playing cup matches, he was called into action when Meslier contracted Covid-19 in January. He also played two matches in May, which now look increasingly like thank you farewell appearances.
Coach Marcelo Bielsa has always been fulsome in his praise for Casilla's goalkeeping and professionalism.
Although the door is not closed completely, it would appear, however, as if his time as a Leeds player is coming to an end.
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