There is a "conditional compulsory purchase option", so if certain criteria are met Villa will pay to make the move permanent, Galatasaray said in a statement.
Zaniolo will likely replace Villa's Argentine midfielder Emiliano Buendia, who suffered a knee ligament injury in training earlier this month and is out for an "extended" spell.
"I'm grateful to have been a part of this beautiful family," Zaniolo posted on Instagram.
"Together we have proven that when you are united, nothing is impossible. Lions family you are now a part of my heart and I promise I'll never forget that."
Zaniolo shone at AS Roma, winning the 2022 Europa Conference League before moving to Galatasaray in February where he played 12 games and scored five goals as they won the title.
Villa were thrashed 5-1 at Newcastle United in their Premier League opener last Saturday. They host Everton on Sunday.
