Neumann has been replaced by Michael Lohscheller, who has been Opel's chief financial officer (CFO) since September 2012. Neumann will remain on Opel's management board until the sale of Opel/Vauxhall to Group PSA, parent company of Peugeot, Citroen and DS, is completed.
The German automaker says this move will ensure a "seamless managerial transition with the priority task to build the new strategic plan for Opel".
Ahead of the Geneva motor show at the beginning of March, the French automaker signed a deal to purchase Opel, Vauxhall and most of General Motors' European operations for 2.2 billion euros ($3.3 billion).
The sale of Opel/Vauxhall was originally expected to close during the second half of 2017. A report in the German press last month indicated the sale process has been slowed down by disagreements over the future of Opel's development centre, its employees and future contracts with General Motors.
Above: Dr Karl-Thomas Neumann.
In a statement announcing his resignation, Neumann said: "It was a difficult personal decision to not continue with the Opel/Vauxhall team when it transitions to Groupe PSA.
"I am proud of the team for all we have accomplished so far and have no doubt that the move to PSA will make Opel/Vauxhall an even stronger and more successful company in the future. I am committed to completing this transaction and will then take some time to decide what is next for me.”
Neumann has headed Opel since March 2013. Prior to joining GM, Neumann had two stints at Volkswagen, where he spent time heading up the company's presence in China, as well as its electric vehicle development team.
He has also held executive positions at Continental, the tyre and automotive components manufacturer.
Sources have told Reuters Neumann could be rehired by the Volkswagen Group as the head of Audi. Rupert Stadler, Audi's current CEO, is reportedly out of favour due to his handling of the company's Dieselgate saga.
Above: Michael Lohscheller.
Opel's new CEO Michael Lohscheller has been with the company since 2012. Before this, he was the CFO of Volkswagen Group of America and Mitsubishi Motors Europe.