Doctors diagnosed Hal with Hodgkin's Lymphoma in May and declared him cancer-free in September. His return will allow the Texans to move Kareem Jackson back to cornerback.
Hal, who started all 16 games last season, will join Tyrann Mathieu and Justin Reid at safety.
The Texans cut linebacker Daeshon Hall to make room for Hal.
Hal returned to practice on Wednesday.
"I knew I was going to play (again)," Hal told ESPN. "I just didn't know when. That's why I never stopped coming. I never stopped coming to practice, I never stopped working out. I just had a feeling that I was going to play again."
After joining the team as a seventh-round pick in 2014, Hal developed into a starter during his second season, starting 38 games from 2015-17. During his career, he's totaled nine interceptions, 26 passes defended and 122 solo tackles.
He earned a three-year, $15 million contract extension before last season.
"He worked very hard," Texans coach Bill O'Brien told ESPN. "(He) did a really good job. He was in all the meetings, let alone the weight room work and the field work. He really stayed on top of the X's and O's.
"He's that type of guy. He's a great teammate. He's a tremendous guy."