In the summer of 2005 Eremenko signed a five-year deal with Udinese. He made his Serie A debut on the first day of the 2006–07 season against Messina. He became the fourth Finnish footballer after Mika Aaltonen, Mika Lehkosuo and his brother Alexei Eremenko Jr to make an appearance in a Serie A match. He was loaned to A.C. Siena on 31 January 2007, where he played 11 games. During the summer of 2007 he returned to Udinese.
In October 2007, Eremenko, Andrea Dossena, Cristián Zapata, and Simone Pepe were awarded a new contract until June 2012.
Eremenko made his international debut in Finland's national team at Helsinki Olympic Stadium on 6 June 2007 in a UEFA Euro 2008 qualification match against Belgium when Roy Hodgson chose him to the starting line up. The game was interrupted for six minutes after a Eurasian eagle-owl intruded the playing pitch. This incident earned the Finland team their current nickname, The Eagle Owls. Eremenko has been a regular member for Finland ever since and he was a key player in Finland's qualification campaigns for 2010 FIFA World Cup, UEFA Euro 2012, 2014 FIFA World Cup and UEFA Euro 2016. He scored his first goal for Finland from penalty spot on 3 March 2010 in a friendly match against Malta.