The shares were purchased at 20 Australian dollar each, bringing ArcelorMittal's total investment in Macarthur Coal to 843 million Australian dollars, a statement from ArcelorMittal said.
It, however, added that the purchase is subject to Foreign Investment Review Board approval.
ArcelorMittal said the acquisition of this holding is in line with the company's strategy to secure supply of raw materials.
Commenting on ArcelorMittal's increased stake in the company, Macarthur Coal Chairman, Keith Lacy said: "We are pleased with ArcelorMittal's continued support and look forward to continuing our long term relationship with them as a major customer."
Lacy said it is "business as usual at Macarthur Coal and we will continue to focus on running and growing the business and delivering increased value to our shareholders".
ArcelorMittal had last month approached Macarthur, the supplier of more than a third of the world's pulverised coal, "in respect of a potential transaction" after acquiring a 14.9 per stake in the coal mining company.
In a regulatory filing today, Macarthur Coal said it is currently making enquiries regarding "rumours that Ken Talbot may sell his shares to Korean steel giant Posco", and will inform the market as soon as it is aware of any relevant information.