The dairy giant already supplies 142 million litres of milk to the society's stores in the south of England and Scotland, and the new contract will add a further 116 million litres.
"The Co-op Group has been an important customer for many years, and we are very pleased to extend our relationship," said CEO Robert Wiseman.
Dairy Crest, which supplies 44 million litres of milk to The Co-op Group's stores in the Midlands, will also deliver a further 29 million litres of milk to stores in the Midlands and Wales, while Yeo Valley will supply eight million litres of organic milk.
The wins come at the expense of Dairy Farmers of Britain, which had a number of milk contracts with The Co-op Group. It is working with the three new suppliers to migrate supply. Dairy Farmers of Britain has had a turbulent few months and this week split its business into two distinct divisions, which analysts said could make it more attractive to potential purchasers.