The 22-year-old has eight Test caps to his name, and joins a Bath influx including former England squad member Duncan Bell from Welsh club Pontypridd.
"Bath Rugby was always the club that I wanted to join, and I am delighted to be signing for them," said Flatman, who is in the England squad to face the Barbarians at Twickenham on Sunday.
"I am really looking forward to be moving to club with great potential and competing for a place in the team."
Bath Director of Rugby Jack Rowell added: "Strong scrummaging is as important a part of the game as ever.
"In addition, prop forwards need to be able to contribute around the field. David's strength, mobility and rugby skills should provide a good impact in the Bath Rugby game next season."
