On 31 August 2012, Ertl joined Portsmouth on a short-term contract. He made his debut a day later, scoring an own goal in a 0–1 home defeat against Oldham
. After playing in centre back and right back roles Ertl finally found his feet in midfield. On New Year's Day, he was handed the captain armband, with Brian Howard
being sidelined by injury. After Howard's departure, he was named permanent captain. Ertl finished the 2012–13 season by receiving the fans' Player of the Season
On 10 June 2013, Ertl signed a three-year contract extension with Pompey. He scored he first goal for Portsmouth on September 7th 2013 in a 2-2 away draw against Cheltenham.