The move has been hailed by the National Farmers' Union (NFU), which drew up the pledge to protect fruit and vegetable growers and improve relations between producers and retailers.
Under the pledge, the UK's fastest growing retailer has made commitments in areas including treating all suppliers fairly, offering greater price certainty to growers, paying them on time and reducing waste.
It also sees Lidl commit to increasing the proportion of British fruit and vegetables available for consumers to buy, when they are in season.
Lidl is only the second supermarket to sign up to the pledge, after competitor Aldi committed to it last year, and farmers hope the move will encourage other major retailers to follow suit.
Ali Capper, NFU horticulture and potatoes board chairwoman, said: "We are delighted that Lidl has publicly committed to our pledge, highlighting its commitment to long-term supply relationships, equitable distribution of reward along the supply chain, and fair and respectful trading relationships.
"Our goal is to generate integrity, honesty and openness across the market and that can only come from the key asks within our pledge - which includes price certainty, transparent working and strong, long-term relationships that are fair for everyone involved.
"Lidl, with its already long history of working closely with its suppliers, is an ideal candidate to make the pledge."
Ryan McDonnell, commercial director for Lidl UK, said: "We are very proud to have developed and maintained strong, long-standing relationships with all our suppliers, and our commitment to the NFU pledge cements this further.
"We're also very keen to ensure that our sourcing process supports the growth and development of UK growers, which is vital in encouraging more and more people, particularly our shoppers, to regularly eat more fruit and veg."
The NFU pledge covers 11 commitments to support growers and help them develop their businesses, focusing on areas including payment, long-term planning, improving the value of the supply chain and direct communication with producers.