The 32-year-old forward arrived at Franklin's Gardens in the summer of 2010. As one of Premiership Rugby's most experienced campaigners he had played more than 200 times for former club Gloucester.
However Eustace's time with the Saints was not as productive and he had a limited number of first team appearances.
Unfortunately Eustace sustained an injury in the neck region earlier in the season, which has now meant that he has been forced to retire - news which he admits has been hard to take.
"When I came to Northampton I wanted to make an impression," Eustace said. "I knew it was always going to be tough with so many good players, but unfortunately I've not been able to fulfil my personal goals.
"I feel really bad and somewhat guilty that I've not been able to repay the club and the fans for their good wishes.
"For the first time in my career I've had one injury after another and it's been very frustrating to say the least. I'm indebted to the club for giving me all the time I've needed to get myself back, but unfortunately I've been given the advice from specialists to stop playing.
"It's been hard to take on the chin, and I'm pretty emotional about it to be honest.
"I'm glad that I was available to play against Gloucester last season, which of course was my old club. And to have been a part of the club when we got to the Heineken Cup final was a fantastic feeling. It's a great squad of players who have been fantastic to me and I'm sorry that I haven't been able to play more with these guys.
"Career-wise my obvious highlight has been making 240-odd appearances for Gloucester, which is my hometown club. And it's been a real pity that I haven't been able to do the same at the Saints.
"I'm going to try to enjoy the Christmas and New Year period as much as I can and then take a bit of time in January as to what's going to happen next. Rugby's been a massive part of my life, and hopefully I can keep my hand in somewhere doing some coaching or community development to try and give something back to the next generation."
Director of rugby Jim Mallinder said he was sorry that Eustace's career at the Saints had finished almost before it had started.
"Adam had shown throughout his career that he was a quality and experienced campaigner and we thought he would bring a great deal to Franklin's Gardens," he said. "While Adam has been a good person to have around the squad and contributed a great deal behind the scenes it is very unfortunate that he has not been able to show the fans his abilities as a player in the first team.
"It is always sad to see a player have his career ended by injury, but we wish Adam all the best with his rehabilitation and every success with whatever he chooses to do in the future."
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