The USA international is scheduled to fly to the UK on Wednesday to tie up a move that will provide extra firepower for the Midlands club.
The 21-year-old is keen to reignite his career after falling down the pecking order under Dortmund head coach Edin Terzic and will get his first taste of Premier League football.
Reyna, son of former Manchester City Sunderland and Rangers star Claudio, hit the headlines a year ago when his mum Danielle became embroiled in a scandal involving US Men's Soccer head coach Gregg Berhalter.
Danielle, a former soccer player, told US Soccer officials about an alleged past domestic violence incident involving head coach Berhalter.
She admitted she was frustrated by comments made about her son by Berhalter after the team's elimination from the 2022 World Cup.
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It was later revealed that Danielle's gripes began after her son did not feature in the the USA's first World Cup game against Wales and began hinting that she had information that could harm Berhalter's career.
US Media claimed her comments were construed as attempted blackmail.
Despite never having played in the Premier League, Reyna is no stranger to English football having spent his early years in the UK.
Reyna was born in Sunderland before moving back to the United States in 2007 where he joined New York City FC's academy.
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In 2019, Reyna moved to Dortmund where he has made 121 senior appearances, scoring 17 times.
The move is a real coup for Nottingham Forest boss Nuno Espirito Santo who hopes the club can sanction multiple new signings on deadline day.
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But Forest will have to tread carefully after a recent charge for a breach of the Premier League's Profitability and Sustainability Rules.
The Portuguese watched on as his side were beaten 2-1 at home to title-chasing Arsenal on Tuesday night.
The defeat leaves Forest in 16th place and just two points above the bottom three as the club brace themselves for a second-consecutive relegation dogfight.
Reyna could find himself thrust straight into the action for Forest's trip to Bournemouth on Sunday.
Following their trip to the south coast, Forest face a tricky run of fixtures with games against Newcastle, West Ham, Aston Villa, Liverpool and Brighton in succession.