The text, found in a computer left in a rubbish bin, has been attributed to Mohamed Abrini, the "man in the hat" seen with the Brussels airport bombers.
He had told investigators he "would not hurt a fly" after his capture in April.
But the text suggests he approved of November's Paris attacks and wished to die a martyr himself, a report said.
Mohamed Abrini had attempted to erase the text found on the computer, placing it in the machine's trash, reported French news channel BFMTV.
It is said to have been discovered in Schaerbeek, a Brussels district believed to have housed hideouts used both by the Paris attackers and suspects who went on to mount the attack on Brussels airport in Zaventem.
Mohamed Abrini's statement was apparently found on a computer discovered in a bin in Max Roos street a day after the 22 March bombings. Investigators have already revealed that the computer contained a statement attributed to Brussels bomber Ibrahim el-Bakraoui.
It is not known why the existence of this latest text has emerged now. The contents of the computer have been key to the Brussels inquiry, but the Belgian federal prosecutor has refused to comment on Wednesday's report.
In the text, dated 2 February, Mohamed Abrini explains that his brother's death in a suicide bombing in Syria in July 2014 drove him towards religion, says BFMTV.
At this point, the 31-year-old Belgian of Moroccan origin decided to join jihadist group Islamic State group, it says.
He describes the perpetrators of the Paris attacks as "heroes", requests his mother's forgiveness, and looks forward to seeing her in "paradise" with his brother. It is signed in his brother's nom de guerre, Abu Yaya.
Mohamed Abrini has been held at Bruges prison since he was dramatically captured in the Anderlecht area of Brussels on 8 April, more than two weeks after the attacks at the airport and a metro station, in which 32 people died.
He is also being investigated in connection with the attacks in Paris that killed 130 people.
He was filmed at a petrol station in northern France with suspect Salah Abdeslam, two days before the 13 November gun and bomb attacks.
Mohamed Abrini's fingerprints and DNA were found on a Renault car used on the day of the murders, investigators have said.
Investigators have discovered a "last testament" written by a key surviving suspect in the March bombings in Brussels, French media say.