Jamie Acourt was detained after he left the Metropolitan Sagrada Familia Gym on Friday, the National Crime Agency (NCA) said.
The 41-year-old, from south London, who is a former suspect in the racist murder of Stephen Lawrence in 1993, is wanted by the Metropolitan Police over his alleged involvement in the large-scale supply of drugs.
Acourt has always denied involvement in the murder of 18-year-old Mr Lawrence in Eltham, south London.
Acourt is to appear in court in Madrid next week for an extradition hearing.
He was apprehended on a European Arrest Warrant (EAW) on Friday afternoon after a joint operation by the NCA, Crimestoppers and the Spanish authorities.
The NCA described him as one of Britain’s most wanted fugitives.
Ian Cruxton, head of international operations for the NCA, said: “Acourt thought he could evade capture but as a result of an intelligence-led operation his days on the run have ended.
“We were able to direct the Spanish authorities to his location in Barcelona.
“Our ability to share information and work at speed with our international partners ensures there is no safe haven for fugitives. We will never stop pursuing these individuals.”
Acourt was the 81st fugitive out of 96 to be tracked down in Operation Captura.
An extradition process can be triggered after an EAW has been issued on an indictable offence which is punishable with at least 12 months imprisonment.
Guidance for Home Office staff on how to deal with the extradition of suspects states: “Extradition is a serious interference with a person’s liberty. You must only request extradition when it is necessary and proportionate because a person may be held in custody for months in the country receiving the EAW while it is decided whether to extradite them.
“The Human Rights Act (HRA) 1998 has protections to safeguard the rights of people who a country is applying to extradite.”
If the suspect agrees to their extradition, and there are no bars to it, the court where they have been arrested will order their surrender and return.
If they do not agree to go back to the UK, it will be up to the court to make a decision on surrender.