The Dutch striker was a prolific scorer for the German side from 2010-17, helping them win the German Cup in 2011. He scored 126 goals in 240 games before joining Ajax in 2017.
But Schalke have fallen on hard times as they are last in the league standings. They broke a 30-match winless streak last week thanks to the sensational three-goal debut of United States youngster Matthew Hoppe, but Huntelaar's veteran presence could provide a boost in the relegation fight.
"We need to win games and score goals to climb up the table," Huntelaar said after competing his move. "I want to play my part in the club staying up.
"Schalke belong in the Bundesliga and it's our responsibility to make sure that we stay here."
The club's head of sport and communication Jochen Schneider added: "There are two main reasons to signing Klaas-Jan. His sporting qualities are undisputed, which he showed once again last week with a brace.
"His personality, experience and connection with FC Schalke 04 are also equally important. He has shown his desire to help us secure safety by returning to the club. I would therefore like to thank Ajax for their cooperation during these talks."
"What do I lean towards? Difficult... There is something to be said for everything," Huntelaar said. "Schalke are in more trouble, they need me more, so that is more of a feeling choice.
"Ajax are a bit more stable and perhaps a bit nicer in terms of ending, with the possible title, the cup tournament and European football."