RTL/ntv and sport.de both report that Dortmund can "well imagine" completing a deal for the 27-year-old, who has been one of the more consistent defenders in the Bundesliga this season.
He scored his seventh goal of the campaign in Saturday's 1-3 defeat against RB Leipzig.
Dortmund need at least one new centre-back after confirming that Niklas Süle will leave when his contract expires in June. Whether Nico Schlotterbeck commits to a new deal is also unresolved, adding further urgency to the club's defensive planning.
Doekhi has confirmed he will leave Union Berlin this summer regardless. Lazio and Wolfsburg were interested during the January window but held back, attracted by the prospect of a free transfer in June.
Leeds United and Aston Villa were also linked earlier in the year.
The Dutchman has made 30 appearances this season and has captained Union through a difficult campaign that has seen them battle to avoid relegation from the Bundesliga. He joined the club from Vitesse Arnhem in July 2022.
No decision has been made by Doekhi on his next destination, though a move within the Bundesliga is under serious consideration. Dortmund's approach is the most concrete to have emerged publicly.