"He's robbed me of everything, my job, my family," Alain Vigneron, a 45-year-old who started work at 14, wrote in a farewell note sent to his trade union representative that was published in the press.
"I've fought for 31 years to have even a small something of my own and now I'll lose my job, and how many other families will lose theirs, Mr Lakshmi Mittal" he wrote.
ArcelorMittal, the world's biggest steelmaker, is cutting thousands of jobs in Europe as the industry struggles to get back on its feet, with 2,000 jobs to go in the Liege area of Belgium where Vigneron worked.
Vigneron added in the note: "I've made placards, I want them in church so everyone can see why I killed myself."
The suicide note topped news headlines in Belgium, which along with France and Luxembourg have been badly hit by job cuts in the steel-making sector.
Management at Lakshmi Mittal's ArcelorMittal said it was "extremely affected" by Vigneron's death and that its social workers would offer his family help.
It refused further comment on "an event touching on private life" in a response quoted by the website of the daily Le Soir.