A source told Ian Rapoport of NFL Network that the deal is for one year and worth up to $6 million.
The move reunites Thomas with offensive coordinator and former Broncos head coach Josh McDaniels. The wideout was drafted by McDaniels and played for him in Denver in 2010.
Thomas has since been to five Pro Bowls and won a Super Bowl, though he's coming off a torn Achilles and pleaded guilty in March to a careless driving charge.
He finished last season with the Texans after a trade-deadline deal sent him to Houston from Denver. In 15 games split between the two clubs, he posted 59 receptions for 677 yards and five touchdowns.
The Patriots have spent the offseason scouring the free-agent and trade markets in search of receiver help. Julian Edelman is the only established pass-catcher set to return from last year's championship team.