A grandfather in his 70s, Al-Marimi has been accused of carrying out the attack along with two Libyans who faced a Scottish court in May 2000.
Three judges heard the case at Camp Zeist, a former US air force base in the Netherlands.
After eight months, they ruled the atrocity was an act of state sponsored terrorism carried out by members of the Libyan intelligence service.
The judges convicted one of the Libyans, Abdulbasset Al-Megrahi, of mass murder and jailed him for life.
He died from cancer in 2012, after being freed on compassionate grounds by the Scottish government.
The Scottish Criminal Cases Review Conviction twice referred Megrahi’s case to the appeal court but the conviction was upheld.
The second Libyan, Al Amin Khalifah Fhimah was acquitted and returned home to a hero’s welcome in Tripoli.
The US government says Al-Marimi admitted bombing the plane along with Megrahi and Fhimah when he was questioned in a Libyan detention facility in 2012.
The alleged confession was handed to Scottish investigators in 2017 and is now central to the prosecution case.
Al-Marimi has claimed the confession is false and was made under duress, but judge Friedrich ruled it can be admitted as evidence at the trial.