The long-time mentor, who is contracted for next season, is set to address his future in a press conference at the Swans' Moore Park headquarters at 1.30pm AEDT on Tuesday.
Sydney's longest-serving AFL coach is expected to confirm his resignation, effective immediately.
Longmire, 53, has been in charge of the AFL club since 2011 after initially joining the Swans as an assistant coach for the 2002 season.
Longmire delivered the Swans the 2012 premiership in just his second year in charge then coached the club to grand final losses in 2014, 2016, 2022 and 2024.
The Swans were thrashed in the final two of those morale-sapping defeats, most recently entering the 2024 decider as hot favourites before being belted by Brisbane in a 10-goal loss.
Highly-rated senior assistant Dean Cox looms large as Longmire's likely replacement.
The Swans have consistently dismissed speculation regarding a succession plan from Longmire to Cox.
But former West Coast ruckman Cox turned down multiple approaches to return to the Eagles as their head coach, in order to remain in Sydney.
Longmire himself was part of a succession plan, taking over from 2005 premiership mentor Paul Roos at the end of 2010.
A champion player at North Melbourne, Longmire kicked 511 goals in 200 games, winning the Coleman Medal in 1990 and a premiership in 1999.
He has coached Sydney 333 times, for 208 wins, three draws and 122 losses, earning an exemplary winning percentage of 62.91 per cent, and missing the finals just twice in 14 seasons.
Of those to have coached more than 150 games, only Chris Scott, Dick Reynolds, Jock McHale and Tom Hafey
It is unclear whether Longmire will step into retirement, look at other football department roles or take a break before returning to coaching.
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