The Voluntary Anti-Doping Association (VADA) reported an adverse analytical finding from Robert Helenius the day before he lost to Anthony Joshua, according to a Matchroom statement.
Matchroom brought Helenius in as a late replacement opponent to face their premier heavyweight Joshua on August 12 at a boxing event in London.
Joshua was originally scheduled to face Dillian Whyte but his old rival failed a drug test.
Joshua drilled Helenius in the seventh round to advance his pro boxing record to 26 wins (23 KOs) against three losses.
In a statement to media which ProBox TV has seen, Matchroom said: "Helenius was voluntarily tested on Friday, 11 August before the heavyweight fighter's defeat by Anthony Joshua in London on Saturday 12 August.
"The result was made known to Matchroom today [Friday]."
It added: "Matchroom defers to the relevant regulatory authorities on next steps.
"We vehemently continue to support voluntary anti-doping testing. We are committed to promoting a clean and fair sport for all athletes."
ProBox TV sought comment from Helenius via social media, and a team member's email, but did not receive an immediate reply.
The statement regarding VADA's "adverse analytical finding" from a Helenius test follows other fighters who have tested positive for performance-enhancing drugs including Whyte, Alycia Baumgardner, and Conor Benn.