Tarrant, 24, will have surgery and miss the rest of the season and likely a large chunk of the 2027 campaign.
The defender, who was All-Australian in 2024, had played all four games in 2026, averaging 2.3 marks, eight disposals and 2.3 spoils.
"Sydney Swans defender Brenna Tarrant will miss the remainder of the 2026 AFLW season with a ruptured anterior cruciate ligament," the Swans said in a statement.
"Tarrant will require surgery, but that will be determined upon further medical advice in the coming days."
Tarrant is the third AFLW Swan, after Alice Mitchell and Alex Neyland, to suffer an ACL injury this year.
The Sydney local started her AFLW career at Melbourne, but joined the Swans when they entered the league in 2022.
Sydney (3-1) currently sit fourth on the ladder after suffering their first loss of the season against the Western Bulldogs.
The Swans next play crosstown rivals GWS at Henson Park on Sunday.
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