Banks Sadler beat agencies from across Europe to win the France-based company's account, which covers 60 countries.
Alstom, which has a turnover of £16bn and more than 80,000 employees, specialises in the manufacture of trains and power equipment.
Banks Sadler chief executive Leigh Jagger said the business would be managed by the agency's London, Paris and Dusseldorf offices.
"The win reinforces the decision to open our European offices and confirms our ability to offer multinational clients the required expertise and local knowledge in key markets," she said.
Jagger added that the agency had started the year on a high with a new account win in the legal sector and re-appointment by a key financial client following a full market tender.