Stagecoach revealed plans to spend £60 million on new vehicles, including a new model for its Megabus budget coach service.
The order also includes 19 hybrid-electric buses destined for Perth and Aberdeen, and supported by a £1.8m Scottish Government grant.
Les Warneford, managing director of Stagecoach UK Bus, said: "This latest multi-million-pound investment is part of our commitment to give local communities high-quality, good value bus travel."
Bill Simpson, corporate affairs director at Alexander Dennis, said: "Coupled with very significant orders we have won in recent weeks in Asia, New Zealand and North America, this gives us a very strong order book for 2012 and a platform to grow our business still further at home and abroad in 2012 and beyond."
The order is also a boost for the remainder of Alexander Dennis's 2000-strong UK workforce.
The firm will be involved in building 226 of the new Stagecoach vehicles. This includes constructing the body for 11 two-tier 72-seat coaches, which are designed to make it easier for wheelchair users to board, and will be introduced on Stagecoach's intercity Megabus services.
Stagecoach said yesterday that it could not give a breakdown on which routes the vehicles could be used.
Alexander Dennis said it will also build the 19 hybrid-electric vehicles which are due to go into service in Perth and Aberdeen this summer.
The single-decker buses have been purchased through the Scottish Government's Green Bus Fund for £5.3m, including a grant of £1.8m from the fund.
The order is another fillip for Alexander Dennis, which has continued to enjoy strong growth and healthy profits despite the economic troubles of the past few years.
Alexander Dennis is expected to see revenues exceed £400m this year, compared to £170m in 2007 when chief executive Colin Robertson took over.
The Alexander Dennis business was rescued in 2004 by a consortium including Stagecoach co-founders Sir Brian Souter and Ann Gloag, Sir Angus Grossart's Edinburgh-based Noble Grossart merchant bank, and businessman and former Rangers Football Club owner Sir David Murray.
Stagecoach said its total investment in new vehicles for its regional bus operations over the past five years now amounted to more than £370m.
The first of the new vehicles will be delivered from May 2012.