MHP will lead media relations for the bus and rail operator, deliver media training and crisis planning services, and provide strategic support for the company's various international PR programmes.
Arriva comms director Alison Cole said: "We were impressed with the MHP team throughout what was an extremely competitive pitch process. We believe they are perfect partners and advisors for us as we look to further growth both in the UK and across mainland Europe. They are well connected and have a strong grasp of the issues and opportunities surrounding the transport sector."
Joshua Peck, managing director at MHP Communications, said: "The company already has an excellent reputation, and we’re looking forward to building on these strong foundations with our sector-leading transport expertise."
Arriva runs over 21,000 buses, 715 trains and 221 trams across 14 European countries, adding up to more than 2.2 billion passenger journeys per year.
In the UK it operates 5,900 buses and 327 trains, has 24,800 employees and became a subsidiary of Deutsche Bahn, one of the world’s leading passenger and logistics companies, in 2010.