The restaurants are the 49th and 50th in the UK to have been awarded the accolade in the past 12 months.
The SRA was particularly impressed by waste reduction initiatives such as doggy boxes and owner Sam Harrison's work to promote healthy eating in schools. It also cited their commitment to serving a high proportion of locally sourced food, a significant amount of free range and organic animal produce and sustainable seafood.
SRA managing director Mark Linehan said: "Sam's and Harrison's are excellent examples of restaurants that sit at the heart of their neighbourhoods, responding to consumer demand for a more sustainable dining experience and playing an active role in the community."
Sam Harrison, who owns the restaurants in partnership with Rebecca Mascarenhas and Rick Stein, said: "It has taken a lot of work from both teams and I am very proud of what they have achieved."
To achieve a sustainability rating, restaurants must provide answers and evidence to 65 questions across 14 areas of sustainability, including whether they are serving local, seasonal, high welfare food, how their resources and waste are managed, as well as how they treat staff, customers respected and the community.