Some Belgian media say he is Najim Laachraoui, thought to be the man on the right in a CCTV image taken before the Zaventem airport attack.
Two of the other attackers have been named in Belgian media as the brothers Khalid and Brahim el-Bakraoui.
Twin explosions at the airport and another at a metro station on Tuesday left about 34 dead and 250 wounded.
Belgium is observing three days of national mourning and held a minute's silence for the victims at midday (11:00 GMT).
So-called Islamic State (IS) has said it was behind the attacks and warned that more would follow.
There has been no official confirmation of the latest arrest or the identity of any of the attackers.
Newspaper La Derniere Heure said Laachraoui was detained by the police's counter-terrorism unit in the capital's Anderlecht area on Wednesday.
Analysts say Laachraoui is believed to be a key bomb maker, and French media say he also played a major role in the terrorist attacks in Paris in November that left 130 people dead.
First victim named
Earlier the RTBF broadcaster identified the el-Bakraoui brothers, saying that Brahim was a suicide bomber at Zaventem airport, where about 14 people died, and Khalid was the suicide bomber at the Maelbeek metro station, where some 20 people were killed.
It said the brothers were known to police and had criminal records.
The broadcaster, quoting a police source, said that Khalid el-Bakraoui, 27, had used a false name to rent the flat in the Forest area of the Belgian capital where police killed a gunman in a shootout last week.
It was during that raid that police found a fingerprint of Salah Abdeslam, the main suspect in the Paris terror attacks of 13 November.
He was arrested in a raid in Brussels last Friday and is due to appear before a pre-trial court on Wednesday.
Khalid el-Bakraoui is on the Interpol website. It says he is being sought for terrorist activities.
RTBF said Khalid was jailed for five years in 2011 for car-jacking while Brahim, 30, was sentenced in October 2010 for firing at police.
Brahim el-Bakraoui is believed to be in the middle of the CCTV image taken at Zaventem airport and released by police.
He and the man on the left were believed to have been killed in the airport attack.
The first of the victims to be named is Peruvian Adelma Tapia Ruiz, 37.
She had been at Zaventem airport with her Belgian husband and twin four-year-old daughters, who were unharmed, her brother told Peruvian radio.
Belgian Prime Minister Charles Michel said Tuesday was "a day of tragedy, a black day".
The country has raised its terrorism alert to the highest level.