The Montenegro Football Association said he "died suddenly early this morning", calling him a "beloved member of the 'Brave Falcons', an eternally smiling guy."
Grimsby-born Sarkic played nine times for Montenegro. He made his debut against Belarus in 2019 and started in goal in a friendly against Belgium on June 5.
His twin brother Oliver is a midfielder with Montenegrin champions Decic Tuzi.
He started his club career in the youth academy at Anderlecht before moving to Aston Villa in 2015.
He had loan spells at Wigan, Stratford and Havant & Waterlooville in England, and Livingston in Scotland, before joining Wolves in 2020. After further loan spells at Shrewsbury, Birmingham and Stoke, he moved to Millwall.
"Millwall Football Club is completely devastated to announce that Matija Sarkic has passed away at the age of 26," his English Championship club said in a statement.
"Sarkic, The Lions' number one goalkeeper, made 33 appearances for the club since joining from Wolverhampton Wanderers in August 2023."
"Everyone at the club sends their love and condolences to Matija's family and friends at this immensely sad time."
The Montenegro football federation echoed the sentiment.
"In the jersey of that iconic London club, he left a mark, which was yet to become part of a bigger and more beautiful story. Premature death stopped that and caused great pain to the Sarkic family."
