The society was competing against 17 firms, including Tesco, Asda, Spar, One Stop, Martins, McColls, to clinch the title for 2008/09.
The society runs 39 branches across Lincolnshire, North Lincolnshire and Newark.
Lincolnshire Co-operative’s Post Office Controller Jocelyn Simmons, said: “We feel that providing post office services is part of our role as a community retailer so we are delighted to have won this prestigious title. This award shows our staff make a real effort to find out what customers need and advise them on what we can offer with a high level of service and expertise.
“It is a testament to all of our hard-working staff in post office branches across Lincolnshire Co-operative’s trading area. We want to thank each one of them for their outstanding efforts during the year.”
The society’s Chief Executive Ursula Lidbetter said: “Our loyal customers also deserve a big thank you. By using our services, they’ve helped us win this award and the support they give also helps us keep our branches open.”
Post Office Ltd Senior Account Manager Dominic Williams commented: “This is a well-deserved honour for Lincolnshire Co-op — an energetic, enterprising and engaged partner.”