Vermaelen is a Belgian centre-half and occasional left full-back for English Premier League side Arsenal. He began his career in Germinal Beerschot's youth system, but moved to Dutch side Ajax's youth academy in 2000.
Rising through the Ajax system, he made his senior debut in 2004 aged 19, but was sent on loan to RKC Waalwijk the following season. Returning to Ajax in 2005, he established himself in the first team with a string of strong performances that won the club the Dutch Cup in 2006 and 2007. Arguably Vermaelen's best season with Ajax was the 2008-2009 campaign, where he captained the side for 6 months and scored five times in 40 appearances.
Ajax were in a transition mode however, and Vermaelen was sold to English side Arsenal in 2009 for an eventual €12 million. Given the number 5 shirt, Thomas scored on debut for the Gunners and impressed as he became known as a goal-scoring defender with eight goals in his first campaign. Unfortunately, a troubling Achilles injury ruined his 2010-2011 season, only appearing five times. However, in 2011 Vermaelen was appointed Arsenal vice-captain in only his third season at the club - a sign of the regard manager Arsene Wenger holds him in.
Vermaelen was called up to the full Belgian side for the first time in 2006, debuting against Luxembourg aged 20. He has been ever present for the side since, making 33 appearances. His solitary goal for his country came against Hungary in a friendly in 2009.