The exclusive contract, which comes after ADL supplied 50 double-deck buses to National Express last year, means the company will be the sole manufacturer for the operator for the next five years.
It will also supply replacement parts for the vehicles.
The ADL site in Falkirk, where 900 staff are employed, will deliver 50% of the contract with the manufacture of double-deck buses, with the remaining half handled at its facility in Scarborough.
The Yorkshire site will manufacture midi buses.
National Express said the new vehicles would join its fleets in Dundee and the West Midlands over the course of the contract.
A spokesman for ADL said the new contract, allied to other business wins with FirstGroup and Stagecoach, had allowed the company to add 50 staff to its roster, with around half in Scotland. He also said the National Express deal will support its ongoing training programmes for graduates, apprentices and staff, noting that one in 10 of ADL's staff in Scotland are engaged in a course to upgrade their skills at any one time.
The company, which has its largest manufacturing base in Falkirk, employs around 2300 staff.
Colin Robertson, chief executive at ADL, said: "We are absolutely delighted to take our business relationship with National Express to a new level.
"Multi-year contracts make so much sense for everyone involved. They give both parties a clear profile of expectations, forward build programmes and financials.
"An added bonus is that they release our best and brightest people, on both sides, to pursue new business growth opportunities, rather than being locked into months of repetitive tender negotiations annually.
"My instinct is that in the years ahead we will see changes in the way we do business and more 'partnerships' of this nature developing in the bus industry."
Peter Coates, managing director of National Express, said: "By working exclusively with ADL we can harness the vast wealth of knowledge within our various engineering teams, improve vehicle reliability and performance, and give our customers better service.
"In parallel with this, we have clear visibility in terms of costs, planning and vehicle replacement programmes, as well as the opportunity to work with and to influence ADL in the way they design and engineer the vehicles of the future."