The new entity has 17 offices in 15 cities in 9 countries across Europe, North America and the Middle East.
Richard Glasson, the current CEO of Gyro, will head the combined operation with Rick Segal, co-founder and CEO of HSR, remaining head of the agency's American operation. Richard Perry continues in the role of chief operating officer.
"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," Glasson said. "This merger creates an agency uniquely positioned to meet the needs of our clients around the world, and able to deliver the truly integrated service that the modern world demands."
The combined agency specialises in technology, telecommunications, building products, financial services, capital equipment and life sciences.
GyroHSR's client list includes American Express, Audi, First Data, GE, Hewlett Packard, Hobart Corp., John Deere, Johns Manville, Nokia Siemens Networks, Pitney Bowes, Shell, Sony, T-Mobile, USG and Virgin Atlantic.
Gyro made gross profits of £34.5m in 2008 and was ranked second in Marketing magazine's agency league table this year. HSR meanwhile achieved its best year on record in 2008, according to US magazine BtoB, increasing revenue by 30% and winning accounts such as that for Pitney Bowes. It has expanded its use of online video and social media, for clients such as John Deere & Co.
GyroHSR's headquarters are in London and the North American headquarters are in Cincinnati. The agency has three offices in the UK, five in the US, two in Germany as well as offices in Paris, Dublin, Amsterdam, Madrid, Stockholm and Dubai.