The body has awarded its Chain of Custody Certificate to a fishmonger - Rock-a-Nore Fisheries in Hastings - and two restaurants - Pissarro's in Hastings and The Place in Camber Sands near Rye.
The three firms can now display the MSC's blue eco-label on Dover sole, mackerel and herring landed at Hastings and caught from sustainable fisheries in the English Channel.
The MSC has also launched a plain English guide to the certification process, MyMSC.
MSC chief executive Rupert Howes said: "MSC certification has been seen as a tool for big players to source sustainable seafood. But with lower fees and MyMSC, it's now just as accessible to small businesses."