The midfielder will see out his existing contract, which runs until 30 June 2013, before moving on to pastures new at the end of the current campaign.
"We were hoping that Lewis would see a future at FC Schalke, but he's made a decision for personal reasons that we respect," explained sporting director Horst Heldt. "We have no doubt that he'll give everything up until the final game of the season and fulfil his duties for the success of the team."
Holtby moved to Schalke in 2009, but it wasn't until 2011/12 that he really made his mark at the Veltins Arena following a fruitful loan spell at Mainz 05. The 22-year-old has made 17 appearances so far this term, scoring three goals.