The 37-year-old has abandoned a title chase with the Dutch giants to help his former club in their battle against relegation.
Schalke are bottom of the Bundesliga table with just a solitary win to their name all season and have made four managerial alterations already this campaign.
Huntelaar previously enjoyed a seven-year spell at the Veltens Arena between 2010 and 2017, scoring 126 goals in 240 games for Schalke.
The former Real Madrid and AC Milan striker won one DFB-Pokal in his first spell at Schalke and is their all-time top scorer in European competitions with 31 goals.
Schalke will be hoping Huntelaar’s return will provide them with the goals to climb up the table. Despite playing a supporting cast for Ajax in the first half of the campaign, Huntelaar chipped in with seven goals in 14 appearances.
"We need to win a lot of games and in order to do that we have to score goals to climb up the table," Huntelaar, who will now wear the number 21 shirt, told Schalke's official website.
"I want to contribute my part to helping us avoid relegation. Schalke belong in the Bundesliga. Now it's up to us to make sure we stay there."
Schalke manager Christian Gross added: "When you've played for Schalke, you're infected by this Schalke virus. He's said himself that he looks back fondly on his time here, and has very, very good memories."
"My feeling from him is that he would love to take on this challenge, and that deserves a lot of respect.
"The man, his personality, his past at Schalke - I'm convinced he can help us in many ways, and he could certainly help [Matthew] Hoppe too – he could definitely learn a lot from him. It would do us the world of good."
Huntelaar announced towards the end of 2020 that he will retire at the end of the season.
