Delph began his career at Elland Road before joining Villa in a £6.5 million deal in 2009, before being beset by injuries and limited to just 36 appearances in two and-a-half years.
Leeds manager Simon Grayson told the club's official website: "As everyone knows Fab is a talented footballer who did very well here and secured himself a big move on the back of what he did.
"This is a great deal for both parties. Fab will get some football and he is the calibre of player that we've been looking to bring in here."
Delph will go straight into the Leeds squad for their clash with Ipswich Town on Saturday and will stay at Elland Road until the 25th of February.
He has re-joined the club as club captain Jonny Howson looks set to depart with a deal to Norwich City reportedly imminent.
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