During the transfer period on the summer of 2009 there were rumors that Hallenius was close to sign for Örgryte Instruction Section and several news papers went out with the news that Örgryte Instruction Section had signed him, which was denied by the club Although the fact was that the club had made an agreement with GIF Sundsvall, but Hallenius hesitated about the transfer. On 7 July 2009 Hallenius signed a 3,5-year contract with Hammarby IF in the Allsvenskan.
During the autumn of 2009, Hallenius made 8 appearances of which he were playing 4 as a starter, but no goals were scored.
Hammarby IF got relegated to Superettan and during the first half of the season 2010, Hallenius has made a huge success, scoring 11 goals in 13 games, including an astonishing Marco van Basten lookalike goal away against Syrianska Football Club on 20 June 2010. This goal was voted second in the.
Since his breakthrough in Superettan, Hallenius have been linked with several clubs during the summer of 2010 that has showed interest in signing him. Apart from Genoa, who signed him just before the deadline, two Dutch clubs including AFC Ajax, and the German giants Bayern Munich has let their scouts follow Hallenius closely.
On 1 September 2010, Hallenius signed for Genoa who immediately loaned him back to Hammarby for the rest of the 2010 season.
He joined up with his new teammates on 4 January 2011.