[4] He played for German national youth teams from 2001 until 2009, winning the 2009 UEFA European Under-21 Football Championship.

Despite starting out as a central defender, he was quickly moved to the forward position and finished as the team's top scorer in 2005–06, with twelve goals in 23 appearances.

With the return of Felix Magath, he found his form and in the 2011–12 season, he showed himself as a key player by scoring and assisting several goals for his club.

[10] After Dejagah stated a desire to join Fulham FC in order to fulfill his childhood dream of playing in the Premier League, Magath stated that he wanted Dejagah to stay at Wolfsburg; however, he later sided with the player's wishes to play at Fulham.

[12] On 31 August 2012, Dejagah joined Fulham on a three-year deal with an additional year's option, reportedly for €2.5 million, reuniting with his former Wolfsburg teammate Sascha Riether.

[4] He made his league debut on 20 October, in a 1–0 home win against Aston Villa, coming off the bench for Fulham with 20 minutes remaining.

[14] He made his first start at Arsenal on 10 November, less than a month after his debut, in a 3–3 draw, while his first assist came on New Year's Day to Dimitar Berbatov in a 2–1 win at West Bromwich Albion.

On 4 December, Dejagah scored his first goal for Fulham against Tottenham Hotspur in René Meulensteen's first game as manager.

[16] On 14 January, Dejagah scored a goal and provided an assist to Darren Bent in an FA Cup match against Norwich City.

[17] On 22 February, Dejagah scored at West Bromwich Albion in his manager's first game again, this time being Felix Magath, his former coach at Wolfsburg.

[18] On 15 March, Dejagah scored the winner against Newcastle United, eight minutes after coming on as a substitute, for his fourth Fulham goal.

Shortly after joining Wolfsburg however, Dejagah suffered a hamstring injury and was sidelined for an extended period of time.

[29] Dejagah made his competitive first team debut on 25 May 2017 in the first leg of the relegation play-off against Eintracht Braunschweig.

On 31 January 2018, Dejagah joined EFL Championship club Nottingham Forest on a short-term deal until the end of the 2017–18 season.

[37] In October 2007, Dejagah refused to play an under-21 international match for Germany against Israel, in Tel Aviv.

[5][41] After a meeting with Theo Zwanziger, the president of the German Football Association, and national technical director Matthias Sammer, Dejagah stated that his reason was that he was concerned because of his Iranian origins about the welfare of his family and relatives if he were to play in Israel; therefore, he was not excluded from Germany's national team.

[7] It had been suggested that Dejagah might play for Iran national team at the international level rather than Germany, but he said he never really considered it an option.

[56] During his time with Fulham, Dejagah resided in London, England, with wife and daughters[44] whose names are inscribed on the external surfaces of his Nike Mercurial boots.