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The Highlanders and Tasman loose forward has had a career littered with injuries, and the 30-year-old has decided to step away from the game following medical advice.
Squire missed most of this year's Super Rugby Aotearoa after a recurring knee injury resurfaced, ruling him out for the season after just two outings.
The 23-test All Black had knee and hip surgeries last year, and returned to the Highlanders after cutting short a two-year deal with the Red Hurricanes in Japan.
"This isn't the post I thought I would be doing so soon but as some of you are aware I have been battling with a knee injury which I've had surgery on," he wrote.
"After 6-7 months full rest to see if it will heal and speaking to my surgeon and [medical] team I have been advised for my best long term interest to retire from all rugby immediately. As disappointing and frustrating as this is, my long term wellbeing has to come first.
"I want to thank everyone that has supported me throughout the years especially the [Highlanders] and [Tasman] for the chance to wear their jerseys. These are two teams I hold close to my heart and I've always loved going to battle with them. Also, the chance to represent my country and wear the [All Blacks] jersey is something I will forever cherish."
Squire's injury troubles are particularly cruel given he spoke of his desire to regain his place in the All Blacks squad following his return to New Zealand.
Squire had been a key member of the team and was widely regarded to be the All Blacks' best player at No 6.
Before the 2019 Rugby World Cup, Squire made the decision to withdraw from the All Blacks team and miss the tournament in Japan, saying at the time that he didn't feel "physically or mentally ready" for the pressures of test rugby, after struggling with similar injuries.
His open discussion of mental health was widely praised in rugby circles.
"Thank you to my family and friends for your unconditional support through the good and bad," he added in his announcement on Instagram. "I couldn't have achieved what I have without you all. Also the coaches, management staff and fans I have met through this journey thank you!
"I am excited for the next chapter in my life and stepping away from rugby … Thank you."