Gennady Golovkin will face Vanes Martirosyan at the StubHub Center in Carson, California, on May 5 in a replacement bout for the originally scheduled clash between GGG and Canelo Alvarez.
According to Kevin Iole of Yahoo Sports, Golovkin will defend the WBA (Super), WBC, IBF, and IBO middleweight titles in the headline fight on a non-pay-per-view HBO card.
GGG's rematch with Alvarez was cancelled April 3 after Canelo failed a pair of drug tests.
Alvarez tested positive for the steroid clenbuterol, which he attributed to his eating contaminated Mexican meat.
Golovkin's promoter, Tom Loeffler, explained the rationale behind selecting Martirosyan as the replacement to Iole:
"Vanes hasn't been active, but he had been looking for a fight. He's been training, and he's interested in fighting GGG. We all would rather be getting ready for the fight with Canelo, but there is nothing we could do about that. Vanes dropped [Demetrius] Andrade; he lost a close decision to [Jermell] Charlo that a lot of people thought he won and he lost a competitive fight with [Erislandy] Lara."
The 31-year-old Martirosyan hasn't fought since losing to Lara in May 2016.
He is a former United States Olympian who boasts a career record of 36-3-1. Martirosyan is also a former light middleweight champion who has primarily fought in that weight class throughout his career.
The 36-year-old Golovkin is 37-0-1 as a professional with 33 wins coming by knockout.
His September 2017 fight with Alvarez marked his career's first blemish, as the bout was ruled a split draw.