The Danish defender had agreed to continue his career at the Greens after Panathinaikos reached an agreement with Aalborg in an undisclosed fee.
Thelander successfully passed the medical and this morning he put pen to paper until 2018.
“I'm very happy and proud to become a Panathinaikos player. I'm looking forward to meet my new teammates and to kick start the season. I'm joining a great club with huge tradition and passionate support. I'm really looking forward to the first game. I hope to do well and help the team to win titles as long as to make the next step in my career,” Thelander underlined after signing his contract.