The soldier was picked up at his barracks in the southern town of Castres on Wednesday morning, police sources said. His arrest follows the detention for questioning late Tuesday of two other men.
Three of Merah's victims were French paratroopers, whom he said he had targeted because of France's involvement in the NATO intervention in Afghanistan.
He subsequently killed a rabbi and three Jewish schoolchildren before being shot dead by police in a siege of his flat in Toulouse.
Detectives looking into the case are convinced that Merah, who had travelled to Pakistan in 2011, could not have acted alone and fear accomplices still at large could represent a security threat.
To date, the only person charged with helping him is his brother Abdelkader Merah, who has been in custody since last year but denies involvement in the killings.
Five other people have been detained and interrogated by police as part of the probe but they were all released without charge.
Since Merah's death, it has emerged that he had been known to France's security services for several years and it has become clear the threat he posed was disastrously under-estimated by agents who had contact with him.
Elsewhere on Wednesday, France's Interior Minister Manuel Valls praised Belgian police for their cooperation after officers shot dead a suspected terrorist after a tip off from France.