Stars is the retail division of publisher Claverley Company and operates 150 stores in the Midlands, the south west and the Home Counties, employing 3,000 staff.
Last year it was ninth in The Grocer Top 50 ranking of independent retailers, with sales of £81m.
The deal, which is subject to pensions regulator approval, will give Midcounties Co-op a portfolio of 297 stores, including supermarkets, convenience stores and newsagents. It will take ownership of the stores in March.
"This represents a very important acquisition for the society. It will greatly increase our presence in the convenience store market and gives us a significant entry into newsagent retailing," said Midcounties Co-op CEO Ben Reid. "Stars is a strong business which will benefit further from being part of a larger retailer like the co-op."
In January last year Stars News Shops left Spar in a bid to refocus on its CTN-based roots. In April it switched to Palmer & Harvey McLane for supplies of ambient goods and to Kerry Foods Direct to Store for chilled food.