Back at Huish Park seven years after originally joining on loan, the 26-year-old signs a two-year deal after a successful period away from Somerset.
Having made his EFL debut in green and white in 2012, the versatile midfielder has gone on to win promotion at all three of his last clubs.
League Two titles were lifted at Northampton and Luton where he made over 100 appearances at both clubs combined, before making it a trio of promotions at MK Dons last season as they secured a return to League One.
D’Ath will now look to add to the 14 appearances made under Terry Skiverton whilst on loan from Reading, taking the number eight jersey.
Reinforcing the middle of the park, manager Darren Sarll said: “Lawson is a player like Rhys Murphy and Lee Collins that I’ve tried to sign on numerous occasions before. The facts speak for themselves that he has three promotions in the last four years and we have capitalised, I believe, on a huge error by league clubs that they have not secured this serial winner.
“Winning comes in many forms and Lawson’s is very evidently his winning mentality, along with his intelligence and creativity. I think Lawson can be a show stopper for the club and he can claim more personal and collective success with us.”