Stagecoach is understood to have paid nearly £10 million for Norfolk Green which employs more than 160 people to operate commercial, tendered and school bus services using 70 vehicles on a network covering Norfolk, and parts of Cambridgeshire and Lincolnshire.
Stagecoach already runs some 360 buses and employs 950 people in the region.
Stagecoach UK bus managing director Robert Montgomery said: "Norfolk Green is an award-winning, high-quality bus operator.
"The business is well-run and has been successful in attracting more people to the benefits of bus travel.
"The acquisition will allow us to expand our operations in the east of England, where we have achieved good passenger growth by focusing on good value fares, investing in our networks and delivering punctual and reliable services."
Norfolk Green's facilities includes a leased depot and bus station in King's Lynn.
It was bought by its current owners in 1996 when it had just three coaches.
The company was put up for sale following the retirement after ill health of main shareholder Ben Colson.
Mr Colson said: "We have built a strong brand in the area with a larger network of bus routes than was here in 1996, attracted new customers to using the bus and increased employment.
"The company goes forward into new hands in good shape, and will benefit from being owned by a major transport company."
The business will be managed by Gary Nolan, regional director north at Stagecoach UK Bus.