The Patriots previously hosted Thomas on a free-agent visit last week. The team didn't release financial details of the contract.
Denver traded the veteran wide receiver to the Texans just before last year's trade deadline. The 31-year-old wideout suffered a season-ending torn Achilles tendon in December and was eventually released by Houston in February.
Thomas had 23 receptions for 275 yards and two touchdowns in seven contests with the Texans last season. Between both teams, he recorded 59 catches for 677 yards and five receiving scores.
The Broncos originally drafted Thomas in the first round of the 2010 NFL Draft out of Georgia Tech. He has played in 132 career regular-season games (118 starts) and tallied 688 catches for 9,330 yards and 62 touchdowns.
Thomas was a member of Denver's Super Bowl 50 championship team. In 10 postseason contests, he has 53 receptions, 759 yards and six touchdowns.
Thomas joins Julian Edelman, who earned Super Bowl MVP honors last season, in the Patriots' offense. New England's receiving corps also consists of Phillip Dorsett, Maurice Harris, Bruce Ellington, Damoun Patterson and Braxton Berrios.
The Patriots also announced the re-signing of restricted free-agent defensive back Jonathan Jones. He originally joined the Patriots as a rookie free agent out of Auburn in 2016. He has spent his entire NFL career in New England.
Jones has played in 48 regular-season games (10 starts) and has recorded 92 total tackles, 2.5 sacks, four interceptions and 16 passes defensed.