Sullivan, who is on the verge of his 37th birthday, will be at Donny until 22nd April.
The veteran was on loan earlier in the season with Doncaster, with his November-December spell culminating in three games.
However, Sullivan has fallen down the pecking order at Leeds - with the return to fitness of Graham Stack as well as the signing of Casper Ankergren on loan.
Sullivan is now set to go straight into Doncaster's squad for the Tuesday clash with Crewe.