Off-contract at season's end, Redmayne's future had been uncertain following the re-signing of young shot-stopper Harrison Devenish-Meares.
It is unclear whether the Gosford-born Redmayne will continue his playing career in Australia.
Redmayne, whose shootout heroics in 2022 secured a World Cup berth for the Socceroos, has only played seven matches this season.
An injury to Redmayne opened the door for Devenish-Meares in December, who was then rewarded with a one-year contract extension for the 2026/27 season on top of his already existing deal.
Redmayne has been a fixture at the Sky Blues since joining from Western Sydney in 2017, tallying 237 games ahead of the club's final three rounds of the home-and-away season.
He is third in the club's all-time appearances record, behind captain Rhyan Grant and former skipper Alex Brosque.
The club confirmed the 36-year-old's departure on Friday ahead of their final home game at Allianz Stadium against Newcastle.
"These eight and a half seasons have been the best of my career, and the most successful. Sydney FC will always have a special place in my heart," Redmayne said in a club statement.
"From the moment I arrived this club welcomed me and gave me everything - belief, support and the chance to be part of something truly special.
"I'm excited for the next chapter of my life and the new adventure ahead, but I will always look back on my time with Sydney FC with immense pride and gratitude."
An emotional Grant said his teammate will leave Sydney as a club legend.
Redmayne, in 2022, became the club's most successful goalkeeper, surpassing Clint Bolton when he claimed his 51st clean sheet following their 2-0 win over Perth Glory.
He has a total of 64 clean sheets.
His decorated stint at the Sky Blues also includes three premierships (2016/17, 2017/18, 2019/20) and three championships (2017, 2019, 2020), alongside two Australia Cup victories (2017, 2023).
"If you just look at what he's done since he's been at the club, it's pretty freakish, so he leaves as a Sydney FC legend," Grant said on Friday.
"It's been a difficult season, especially the second half of this season so it'll be pretty upsetting to see him go.
"I've known 'Redders' for 18 years, maybe a bit longer.
"Not only is he a really good teammate, he's a really good mate of mine. He's my roommate, has been for a number of years, so it's going to be really sad to see him go."
The four-cap Socceroo danced his way into Australian football lore in 2022 as the star of the penalty shootout that sent Australia to a fifth-straight World Cup finals.
Thrown into the fray late as a back-up to Maty Ryan, Redmayne denied Peru's Alex Valera with the Wiggles-style dance moves he had also displayed in the 2019 A-League grand-final win over Perth.
"For a Sydney FC point of view, obviously against Perth in the grand final," Grant said of his defining moment.
"But as a good friend of 'Redders', I think the moment he had with the Socceroos, where he was the hero to get him to the World Cup - you can't go past that."
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