Europe's steel industry is struggling with long-term excess production capacity, while the economic downturn has hit demand.
The decision comes days after ArcelorMittal bowed to pressure from the French government to invest in a site in north-eastern France to avoid forced layoffs.
The Belgian investment, which should help save 2,000 jobs at ArcelorMittal's plant in Liege, eastern Belgium, had been suspended after the steelmaker failed to come to an agreement with unions over details of job losses.
However, the company has now agreed the plan with unions and Jean-Claude Marcourt, minister for economy in the Walloon regional government, it said in a statement emailed on Wednesday.
The investment should help retain activities at the plant, such as rolling steel and research and development.
ArcelorMittal is closing down two blast furnaces and a foundry at its Liege site, with unions estimating that 500 jobs would be lost.