The deal is the biggest contract win for Primesight since private equity company GMT Communications Partners acquired it from SMG for £62m last August.
Primesight, which already holds point-of-sale contracts with Co-op food stores, One Stop and Texaco, will now be able to offer advertisers convenience store campaigns on up to 2,000 panels. Existing customers on the point-of-sale format include Nestle, Coca-Cola, Evian, Cadbury and Danone.
Somerfield is now part of the Co-op group after the Manchester-based retailer paid £1.56bn for it last month.
The contract for Somerfield's point-of-sale advertising was previously shared between Clear Channel, Titan and Primesight.
As part of the agreement, Primesight will upgrade sites outside parts of Somerfield's existing 1,000-strong portfolio of UK stores, which have an average weekly footfall of 12 million customers.
Primesight said that as part of its package it regularly mailed store managers about upcoming point-of-sale campaigns to encourage better shelf positions for advertised products.
Nigel Clarkson, Primesight group sales and marketing director, said the partnership "strengthens our market-leader status in convenience advertising as Somerfield is by far the biggest convenience retailer in the UK".
The contract win comes just a few weeks after Primesight announced its expansion into the shopping mall market with a new four-by-one-metre format. Primesight Mall2 went live on June 30 in 20 UK shopping centres, including Bluewater in Kent.
The company now plans to expand the Mall2 portfolio to 200 sites, 90% of which will be in the UK's top 50 shopping centres.