Italy advances to Nations League quarter-finals
Tonali – who recently returned to action after serving a 10-month ban for breaching betting rules – scored his first senior international goal in the 11th minute.
He fired home after Giovanni Di Lorenzo's cutback deflected off Zeno Debast and fell kindly into his path at close range.
Luciano Spalletti's visitor continued to enjoy the better of the play in the first half, with Nicolo Rovella going close when he fired high and wide in the 25th minute.
Mateo Retegui was denied by Koen Casteels when one-on-one after half-time, while Gianluigi Donnarumma was called into action at the other end by Debast and Lois Openda.
Belgium's best chance for an equaliser fell to defender Wout Faes seven minutes from time, but he headed against the post when left unmarked in the centre of the box, leaving the Red Devils locked in a battle with Israel to avoid finishing bottom of Group A2.
Italy has won three competitive away matches in a row within the same season, beating France, Israel and Belgium.
It last achieved that feat between June and November 2019, winning five in a row in qualifying for Euro 2020.