Haas joined Telefónica in 1998 and will take up the reins on 1 January 2017 as part of a three-year contract. He joined the management board in 2009.
Telefónica said he will work closely finance chief Rachel Empey in managing the firm, while Thirks will remain on the board in an advisory capacity until March, before becoming CEO of low-cost airline Eurowings, a subsidiary of Lufthansa.
Eva Castillo Sanz of the Telefónica advisory board said: "We are pleased to announce the appointment of Markus Haas as new CEO of Telefónica Germany. Markus Haas and Rachel Empey have in recent years proved a strong management team. Together, they will drive the company's digital transformation.
“Looking at the ongoing integration and transformation, the corporate occupation of the CEO position is a clear signal of continuity to our shareholders, employees and business partners. The Supervisory Board looks forward to the further successful collaboration between them."