Last month's killing of Tahir Ahmad Nasim in the northern city of Peshawar provoked outrage in the United States.
The US State Department has demanded action in his case and called for a reform of the controversial blasphemy laws under which he was being held.
Nasim, a member of the minority Ahmadiyya community, was under police escort when he was fatally shot in court on July 29.
Blasphemy accusations are highly inflammatory in deeply conservative Pakistan and have in the past sparked mob lynchings, vigilante murders and mass protests.
According to investigating officer Lalzada Khan, junior lawyer Tufail Khan was arrested on Tuesday for "allegedly providing a pistol to the assassin to kill Nasim".
"Tufail was produced before the judge in an anti-terrorism court. He was remanded into police custody for three days," Khan told AFP.
Authorities say 17-year-old Faisal Khan has confessed to the killing, claiming Tufail provided him with the pistol.
Lawyers don't typically undergo a pat-down on going into Pakistan courts, and officials said Tufail had discreetly handed the pistol over to Faisal.
Washington has said Nasim was lured from his home in Illinois to Pakistan in 2018, when he was arrested on blasphemy charges.
The Ahmadiyya have long faced violence in Pakistan, which forbids community members from self-identifying as Muslims.
The sect, which is rooted in the subcontinent and shuns violence, is considered heretical by many orthodox Muslims for challenging the Islamic belief that the Prophet Mohammed was God's final messenger.
Up to 80 people are known to be imprisoned in Pakistan on blasphemy charges -- half of whom face life in prison or the death penalty -- according to the US Commission on International Religious Freedom.
The US State Department has put Pakistan on a blacklist over religious freedom, pointing to the blasphemy cases.
A senior police official told AFP that Nasim held dual Pakistan-US nationality.