"We're absolutely delighted that Will is returning," Saracens director of rugby Mark McCall said in a statement.
"In a very short space of time here his contribution to the side was superb, he showed a real appetite to improve his game and he was very popular among all the players and staff."
Skelton, who has 18 caps for Australia, said his decision to return to England had not been an easy one, not least because it would hamper his chances of being called up by his country.
"I am very excited to be returning to Saracens. This time around I am signing a longer-term deal, so it's been a really big decision and one I haven't made lightly," he added.
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"It's not easy leaving the NSW Waratahs and the Wallabies -- Darryl Gibson and Michael Cheika have been fantastic for my career so far and I want to thank them for everything they have done for me, on and off the field.
"Not being able to pull on the gold jersey while I am playing in England has made the decision especially difficult, but this is about family as much as it is football."
Skelton is Sacarens' fourth signing for the 2017-18 season, and joins Toulouse and France hooker Christopher Tolofua, England flanker Calum Clark and Wales fullback Liam Williams at the club.
(Writing by Simon Jennings in Bengaluru, editing by Mitch Phillips)
