Harrison had been on the NFI since the start of camp and can now begin fully practicing with the team.
Harrison, 30, originally signed on with the Jets as an undrafted free agent out of William Penn back in 2012. He spent four years with the Jets before departing in free agency for a five-year, $46.25 million contract that included $24 million guaranteed with the Giants.
The Giants later traded Harrison to the Lions in return for a 2018 fifth-round pick. Harrison stands to make base salaries of $6.75 million and $9 million over the next two years.
In 2018, Harrison appeared in 10 games for the Lions, recording 50 total tackles, 3.5 sacks, one forced fumble and one fumble recovery. In seven games for the Giants, he recorded 31 tackles, no sacks and a forced fumble.