The deal will see the Revs receive $300,000 in 2022 allocation money and another $300,000 in 2023.
The move is something of surprise given that Lletget was acquired by New England from the LA Galaxy prior to the 2022 season for $500,000 in allocation money, which could have risen to $1.3 million if incentives were met.
Lletget had been a steady presence in the Revolution lineup as well, appearing in 19 of New England's 23 league games -- 18 of them starts -- scoring twice and adding five assists.
"We would like to thank Sebastian for his professionalism and dedication to the Revolution this season," Revs head coach and sporting director Bruce Arena said. "This trade presented a great opportunity for the player and our club, and we wish Sebastian nothing but success in Dallas."
The 29-year-old has a prior working relationship with Dallas manager Nico Estevez from their time with the United States national team when Estevez was an assistant, so the expectation is that Lletget will slot right into Dallas' lineup.
"Excited is an understatement," Lletget said in FC Dallas' announcement. "I'm very familiar with coach Nico Estevez. I worked with him on the national team. I played with several players that are on the on the team now and I just believe in where the club is going. I think it's going be a great fit for me and for the club."
"I've worked with him the last three years with the national team," Estevez said. "He's a great guy for the locker room. He's humble, a hard worker. He gets along with any player of any age and he speaks two languages: Spanish and English. I think this will help him get close to the guys and get along with them."
In parts of eight MLS seasons with the Galaxy and New England, Lletget scored 27 goals and added 32 assists in 183 league and playoff appearances.
Prior to joining the Galaxy in 2015, Lletget was at English Premier League side West Ham United, joining their youth setup in 2009. But he made just one senior appearance during his time with the Hammers and, in a bid for more playing time, moved to Los Angeles.
At international level, Lletget has made 33 appearances for the U.S., scoring eight goals. His World Cup prospects have dwindled in 2022, though, having failed to make an appearance during the calendar year under manager Gregg Berhalter.