The 25-year-old will join newly-promoted Forest on a season-long loan as he looks for regular first-team football.
The United keeper spent most of last season playing second fiddle to David De Gea, and made a mere three appearances for the Red Devils - none of those coming in the Premier League.
Contracting Covid-19 during last year's pre-season certainly didn't help Henderson's cause, as he struggled to make headway in the battle to become United's number one goalkeeper.
This will be the sixth loan move for Henderson since 2015, having featured for the likes of Stockport County, Shrewsbury Town and Sheffield United.
The player will now join a Nottingham Forest side who are preparing for their first top-tier season since 1999.
Steve Cooper's side beat Huddersfield Town in the Championship play-off final last season to clinch their Premier League return.
While the loan deal does not include an option to buy, Forest are understood to be covering Henderson's wages throughout the duration of the loan.
As part of the agreement, the England international cannot play against United when the two teams clash in the Premier League.
Henderson's last season away from Old Trafford took place during the 2019/20 campaign, when he helped Sheffield United to a ninth-placed finish in their first Premier League season since 2007.
With the player hoping to seal his place in Gareth Southgate's World Cup squad, the loan move will provide the keeper with an opportunity to shine ahead of November's tournament in Qatar.