Pools announced Liddle, 35, would be leaving the club this summer bringing an end to his second spell 10 years on from his first exit.
Liddle had been offered terms over a new contract to remain with the club next season under new manager Paul Hartley but the experienced defender, who has represented Pools over 350 times as a player, will make the switch to non-league with South Shields.
Liddle will become the third ex-Pools player to join the Mariners this summer after midfielders Martin Smith and Michael Woods linked up with former Sunderland striker Kevin Phillips at the 1st Cloud Arena last week.
Liddle, who declared recently he would happily retire with Pools, has conceded he always knew his time at the Suit Direct Stadium would come to an end and has thanked fans for their outpouring of support over both spells before wishing the club well for what he describes as an ‘exciting’ time under new boss Hartley.
“I always knew I’d have to leave one day and here I am leaving for the second time,” Liddle wrote.
“I do so with a huge sense of achievement. It’s been a whirlwind two-and-a-half years packed with highs, lows and COVID of course in that first season, but two-and-a-half years filled with a huge amount of success and a personal sense of accomplishment.
“At 33-years-old I was welcomed back to your club and it was like I had never left.
“What transpired over the next 18 months ended with the proudest day of my 15 year career in Bristol.
“To see the club regain not only its Football League status, but also its connection with its fans again was huge.
“We weren’t the best team in the league, far from it, but whatever it was that we lacked on the pitch was soon made up with the support in the stands at the Suit Direct Stadium.
“After league status was comfortably secured last season I hope the club can push on again in the coming season under the new manager.
“It’s hard to leave but I’m also proud to leave the club again as the Football League club it is.
Liddle added: “I should explain that I made this decision before a new manager was appointed and I’d like to wish Paul Hartley every success for the future. It's an incredibly exciting time for Pools.
“Thanks again everyone and good luck for next season and beyond.”
Pools chairman Raj Singh also commented on Liddle’s exit revealing he understands his decision to leave while thanking him for his remarkable service to the club.
“Everyone at Hartlepool United would like to wish Gary all the very best for the future,” said Singh.
“We thank him sincerely for his service to the club. He has been a remarkable servant and a true professional both on and off the pitch.
“We have discussed the club's future plans and in context we understand Gary’s reasons.
“He has an opportunity to join a club on a long-term deal with security for himself and his family at this stage in his career.
“He leaves with our best wishes and he will always be welcome at the Suit Direct Stadium.”