]; born 14 November 1985) is a Belgian former professional footballer who played as a defender. Being left-footed, he usually played as a left-sided centre-back and could play at left-back as well. His key attributes included aerial ability, leadership skills, and his quickness with the ball at his feet. He was also known for his goalscoring ability as a defender.

Vermaelen joined the Ajax youth academy in 2000 and won the 2003–04 Eredivisie title in his first season with the club. He had a loan spell with RKC Waalwijk before rejoining Ajax and winning the KNVB Cup and Johan Cruyff Shield. After Klaas-Jan Huntelaar's departure in 2009, he served as club captain for the remainder of the season. He joined Arsenal in 2009 and was included in the PFA Premier League Team of the Year for that season. Following Robin van Persie's transfer to Manchester United in August 2012, he was named club captain but found first team opportunities limited due to the success of Arsenal's new first-choice defensive partnership of Per Mertesacker and Laurent Koscielny. In August 2014, he completed a £15 million transfer to Barcelona in La Liga. In 2019, he joined Vissel Kobe before retiring in 2021.

Vermaelen made his international debut for Belgium in 2006, and took part at the 2008 Olympics, where Belgium came fourth, as well as the FIFA World Cup in 2014 and 2018 (where Belgium came third), and the UEFA European Championship in 2016 and 2021.