The single-decker GB Kite Hydroliner buses are manufactured by Wrightbus in Northern Ireland and will be delivered in June 2022.
Hydrogen will be stored at the Metrobus Crawley depot in liquid form, before being converted to gas held in tanks on the roof of vehicles.
The single-decker fuel cell buses are funded in part with money from UK Government and European Union zero-emission bus schemes, plus investment from Gatwick Airport. A further possible order of 34 buses could bring the fleet to 54 vehicles, which would be the largest local fleet of hydrogen buses deployed in Europe to date.
As well as having zero carbon emissions, hydrogen buses have long range capabilities and can be refuelled quickly in depots.
Go-Ahead is already the UK's largest operator of zero-emission electric buses, with a fleet of nearly 300 nationwide, but this is the group's first venture into hydrogen. Go-Ahead has set a target of transitioning to a zero-emission bus fleet across the UK by 2035.
Christian Schreyer, Group Chief Executive, The Go-Ahead Group, said: "This is our first hydrogen supply deal, and it represents a milestone in our plans to transition Go-Ahead's entire fleet of buses to zero emission fuel.
"We're delighted to be working with Air Products and with Wrightbus. This is an example of what can be achieved through a mixture of public and private funding, and of co-operation between different environmentally responsible partners."
Manish Patel, Air Products UK Hydrogen for Mobility Director, said: "We're pleased to be partnering with Go-Ahead and Metrobus on the largest hydrogen supply contract for buses in the UK to date. It's a huge milestone in the UK's hydrogen for mobility sector.
"At Air Products, we've been involved in more than 13 million hydrogen vehicle fills across the world and have extensive experience over the last 60 years in delivering reliable end-to-end hydrogen solutions, from production to distribution and fuelling stations. Our innovative and unique solutions will provide Metrobus the infrastructure to not only fuel their initial fleet but allow for rapid expansion without additional footprint or cost."
Ed Wills, Managing Director, Brighton & Hove and Metrobus, said: "We went through a rigorous process before selecting Air Products as our supplier. We were impressed not only with their experience in this field but with the assurance that they would be ready to begin supplying us at volume from day one."