Thirteen received jail terms of between one and eight years, some suspended, according to Belgian news service Belga.
One member of the group was sentenced to community service.
The group are said to have made around 2,000 fake documents.
The documents were used by Salah Abdeslam, a suspect of the November 2015 attacks in Paris, and Najim Laachraoui, who blew himself up at Brussels Airport in March 2016.
The Islamic State group took responsibility for the attacks in both countries.
However, the ID forgers were not prosecuted for terrorism as they were not known to have been aware of the intentions of those who bought their work.
Abdeslam, the main surviving suspect from the Paris attacks, fled France after 130 people were killed in a series of coordinated suicide bombs and shootings.
Following an international manhunt, Abdeslam was arrested in Brussels in March and, the following month, transferred to France, where he has kept silent.
His precise role in the attacks is unclear, although investigators believe he may have driven the three bombers who attacked the Stade de France stadium.
A passport for a 25-year-old Syrian was found at the stadium after the explosions, and authorities later said they believed it to be a fake.
A passport bearing the same name and data - but a different image - was later found on another migrant, suggesting the documents originated from a counterfeiter.
It is not clear if these documents were produced by the same gang.
The Belgium attacks - consisting of two suicide bombs at Brussels Airport and one at Maalbeek metro station - killed more than 30 people and injured hundreds.
A Belgian court convicted 14 people on Thursday of falsifying identity documents, some of which were sold to Islamist militants involved in terror attacks in Paris and Brussels.