Zaid al-Hilli, 54, was picked up in Chessington and was being interviewed, Surrey police said in a statement.
Police said Hilli is the brother of Saad al-Hilli, from Claygate, who was found shot to death along with his wife, Ikbal, her mother Suhaila, al-Allaf, who lived in Sweden, and Sylvain Mollier, a French cyclist, on a remote forest road in Chevaline on Sept. 5.
Officers from the Surrey and Sussex Major Crime Team have been working with French authorities investigating the killings.
"This pre-planned arrest is a result of these ongoing inquiries," the police statement said.
Senate Democrats to pull all-nighter on climate change
Dennis Rodman pledges to end trips to North Korea