The 22-year-old midfielder, who left Simon Grayson's side to join Aston Villa for £6m in the summer of 2009, has being made available for loan by Villa boss Alex McLeish.
Delph was a regular in the Villa midfield in the first half of the season but has not featured for the first-team since mid-December.
Leeds boss 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 made 46 league appearances in his first spell with the West Yorkshire side and will be available to face Ipswich in the npower Championship on Saturday, with the deal due to expire on February 25.