The ground breaking agreement confirms their pay and conditions of employment, including welfare facilities and training.
It follows the signing of similar agreements with unions in June 2013 for civil workers, as well as broader agreement establishing the framework for industrial relations for the project.
EDF and the unions said that having the agreements in place demonstrated a partnership approach that would create a climate for positive industrial relations, promoting safety, quality and productivity.
Unite national officer Kevin Coyne said: "This agreement sets a new benchmark for pay, conditions and apprenticeships. It ensures there are at least 500 well paid apprenticeships on the project which is a massive opportunity for young people to get a skill for life. It also commits to direct employment and no agency workers which ensures the workforce are more secure and enjoy equal treatment."
Hinkley Point C construction director Nigel Cann said: "This upfront collaboration gives us confidence that once we start construction, we will be able to continue without stopping.”
EDF Energy chief executive Vincent de Rivaz said: “Hinkley Point C has the potential to create 25,000 job opportunities in the UK during its construction, including over 400 apprentices, and it will create 900 jobs when operational.
“It is important that these jobs are covered by an agreement promoting the very best standards in industrial relations. I am pleased that EDF Energy, Unite and GMB will work together to provide a high class work force for the construction of Hinkley Point C and help rebuild the nation’s nuclear industry.”