Per team policy, terms of the deal were not released, but reports indicate it is a two-year, $6 million deal.
The 7-footer joins the Nets after spending last season with Valencia in Liga ACB, Spain's top league, where he earned All-ACB first-team honors after averaging 14.1 points and 5.4 rebounds in 22.5 minutes per game in 40 games with the club.
"Justin is an energetic big with the ability to stretch the floor," said Nets general manager Sean Marks. "He is coming off a successful season in one of Europe's most competitive leagues and will add depth to our frontcourt rotation."
Prior to playing in Spain, the 26-year-old appeared in 49 games in parts of two NBA seasons with Charlotte, Miami and Minnesota, registering averages of 5.0 points and 2.9 rebounds in 16.0 minutes per game.