Officers arrested the unidentified man on Monday in Benidorm on Spain's eastern coast less than 24 hours after a European arrest warrant was issued for him, police said in a statement.
Spanish police said he fled to Spain after it was discovered that he sexually abused two brothers aged 14 and 16 on 22 occasions in Britain between 2007 and 2010.
"The fugitive befriended the victims' parents, who allowed him to spend time with their sons, unaware of the sexual acts the boys were subjected to," the statement said.
A British court sentenced the man in absentia to 43 years in jail for six counts of rape and several counts of sexual assault on the minors.
Spanish police had already detained him for possession of a "huge amount" of child pornography which he shared online.
He was charged with possession and distribution of child pornography.