The 21-year-old will spend the rest of the season at Hayes Lane after agreeing to join the Ravens for the 2022/23 campaign.
Hannam featured heavily in Shaun Derry’s Under-23s side which won the Premier League 2 Division 2 play-offs in 2020/21, making 18 appearances. He consequently won the 20/21 Under-23s Player of the Season award and made the first-team bench for several Premier League games.
Bromley are managed by former Palace youth player Andy Woodman and will look to push on in the National League after a 10th-place finish and FA Trophy victory in the previous campaign.
Hannam said: “I’ve really enjoyed my time at Palace since joining two years ago and am now looking forward to this opportunity to play senior football with Bromley.
"Last season was a challenge in my career but it taught me a lot and I’ve worked hard to be ready for this opportunity now. I can’t wait to get going.”