The merged company is called GyroHSR and offers PR, media and relationship marketing, advertising, brand strategy and digital services.
The new entity has 17 offices in nine countries across Europe, North America and the Middle East. Combined revenues total approximately $100m (USD) and GyroHSR employs more than 600 employees.
‘Both Gyro and HSR recognise and embrace the changes that we are seeing in the world of marketing and communications, and this is our response,' said GyroHSR chief executive Richard Glasson.
Glasson will direct the new agency's overall strategy and operations from London.
GyroHSR clients include American Express, Audi and Virgin Atlantic.