With Millwall, the Montenegro goalkeeper died suddenly on Saturday morning, with local media reporting he collapsed at his apartment in Budva.
The Montenegro FA said in a statement: "His untimely death has caused great pain to the Sarkic family."
Grimsby-born Sarkic joined Millwall last August and made 33 appearances last season.
Millwall stated: "Millwall Football Club is completely devastated to announce that Matija Sarkic has passed away at the age of 26.
"Sarkic, The Lions' number one goalkeeper, made 33 appearances for the club since joining from Wolverhampton Wanderers in August 2023.
"A Montenegro international, Matija proudly represented his country on numerous occasions.
"Everyone at the club sends their love and condolences to Matija's family and friends at this immensely sad time.
"The club will make no further comment at this time and asks that the privacy of Matija's family is respected."