The agency won the account in a competitive pitch. Tulchan Communications was the previous incumbent.
The account will be led by Andrew Hayes, chief executive, along with Jessica Rouleau, director, and Maria Ignatova, head of CIS practice.
Hayes commented: 'Severstal is committed to the highest international standards of comms and we are delighted to have been appointed to work with its strong internal team on the next stage of the company's development.'
Olga Antonova is Severstal's head of external and internal comms in Russia and the firm uses Taylor Rafferty in the US.
Listed on the Moscow Interbank Currency Exchange, the Russian Trading System Stock Exchange and the London Stock Exchange, the firm operates key assets in Russia, the US and Europe.
The firm employs a workforce of 92,000 at Severstal Russian Steel, Severstal International and Severstal Resources.