Ali Qazimaj, who was wanted over the murder of a pensioner and disappearance of his wife, has been arrested in Luxembourg, police said.
Peter Stuart, 75, was found with multiple knife wounds in woodland near his home in Weybread, Suffolk, on Friday June 3.
Police are continuing to search for his missing wife Sylvia Stuart, 69, who was last seen on May 29.
Qazimaj, 42, was detained after a member of staff at a hostel in Luxembourg recognised him from a public appeal.
A Suffolk Police spokesman said: "A member of staff working at a hostel in Luxembourg identified Ali Qazimaj through photographs of him on the internet and then alerted authorities who detained him.
"Detectives from Suffolk are now working with the National Crime Agency, the Crown Prosecution Service and authorities in Luxembourg to seek Ali Qazimaj's extradition following the granting of a European Arrest Warrant relating to the murder of Peter and Sylvia Stuart."
Officers have said Mrs Stuart's disappearance "is now being treated as a murder inquiry". It has been more than two weeks since she was last seen.
Police carried out searches between the Stuarts' home and Tilbury, Essex, where Qazimaj lived.
Qazimaj, a Kosovan immigrant who goes by the name Marco Costa, caught a ferry from Dover to Calais at around 6:30pm on Saturday June 4. He was unaccompanied on the journey.
He was living in Tilbury and had been working in Purfleet as a forklift driver.
Mr and Mrs Stuart were captured on CCTV walking into Goodies farm shop in Wood Lane in Pulham Market at around 10.20am on May 29.
A 61-year-old man from Leicester was arrested on suspicion of murder and released on bail to return to Martlesham Police Investigation Centre on August 3.