Beckham, who began training with the team in October last year and joined it in January on a six-month loan from Inter Miami, signed a one-year contract with an extension option for an additional year.
The 20-year-old was part of the squad that in May lifted the Premier League Cup, a competition run by the Premier League for under-21 sides. Beckham, son of former England captain and Inter Miami president and co-owner David Beckham, featured 15 times for Brentford B and scored one goal during his loan spell.
"We added Romeo to our squad in January and he's been fantastic for the group," Brentford B coach Neil MacFarlane said. "We've continued to develop him in and out of possession and we look forward to him coming back in good spirits to continue working with him next season."