The defender spent eight years at St James Park, having joined the Magpies from Spanish club Deportivo La Coruna in August 2008. He made his debut at Old Trafford and later went on to captain the team.
However, following the club's relegation from the Premier League last season, the 34-year-old has opted to return to his homeland.
Coloccini made 275 appearances for United in all competitions, scoring seven goals.
And managing director Lee Charnley told the club's website: "It is rare in football these days that a player gives such length of service to one club and I would like to thank Colo for his contribution to Newcastle United.
"In the eight years he has been with us, Colo has become a firm friend and colleague to many players and staff, past and present, who I know will miss him. We wish him all the very best for his future career."
Coloccini joined Newcastle United on a five-year contract in August 2008 for a fee of up to £10.3 million. Two days later, he made his debut against the reigning league champions, Manchester United, playing 90 minutes in a 1–1 away draw.
Since his debut, Coloccini has been one of the most consistent inclusions in Newcastle's squad, and formed a partnership with fellow newcomer Sébastien Bassong in his first season.
He was not immune to the pressures of representing Newcastle's infamous defence, coming under his most intense criticism for his performance in Newcastle's 5–1 defeat to Liverpool. Despite his mistakes in the game, manager Joe Kinnear voiced his support for Coloccini, stating that he believed his "Mr. Reliable" would bounce back.
On 1 February, Coloccini was substituted against Sunderland in the 93rd minute. Up until this point, he had played every minute of every game for Newcastle all season. He was dropped for the 1–1 draw with Stoke City, but played in Newcastle's 1–0 defeat to Aston Villa, which saw them relegated.
Despite Newcastle's relegation, Coloccini stayed on and was the mainstay of the club's defence all season, partnering the likes of Steven Taylor, Mike Williamson, Fitz Hall and Tamás Kádár in the sturdiest rearguard of the league. He scored two goals for Newcastle since relegation, a header at Cardiff City, and another header at Watford.
The club was eventually promoted back to the Premier League as champions, and Coloccini's great contributions awarded him a place in the Championship's Team of the Year. Coloccini scored his third Newcastle goal against Wigan Athletic in the fourth minute of injury time as they came from two goals down to draw 2–2 at St James' Park, with that being his first goal in the Premier League on the same day that he captained Newcastle for the first time.Coloccini scored a striker's finish in a 1–1 draw with Tottenham Hotspur on 22 January 2011, the day of his 29th birthday and his 100th Newcastle appearance.
He continued his fine form for Newcastle into April, reforming his partnership with Mike Williamson after a brief time with Steven Taylor in January. On 2 April, Coloccini made numerous forward runs and forward roaming to help supply Peter Løvenkrands' goal in the 4–1 win over Wolves. Manager Alan Pardew, along with pundit Jamie Redknapp complimented the defender for his contributions to the team's attack, calling him a "top, top player".
On 8 July 2011, Coloccini was named Newcastle captain for the 2011–12 season. He and Taylor rekindled their partnership and were central to one of the Premier League's meanest defences, conceding a mere 8 goals in the first 11 games. He scored his first goal of the season in the 122nd minute to win the game against Championship club Nottingham Forest in a 4–3 thriller in the third round of the League Cup that saw Forest equalise on three occasions.
In October, he was selected for the North East Football Writers' Association Player of the Year for 2011.[12] On 22 April 2012, Coloccini was named in the PFA Team of the Year. After an impressive display in the Tyne–Wear derby against Sunderland in October, Pardew was so impressed with the way Coloccini played that he said the performance was just like watching England legend Bobby Moore.
In January 2013, Coloccini stunned Newcastle by asking for a transfer back to Argentina citing personal reasons. His preference was to move to the team he helped win the 2001 Clausura, San Lorenzo. However, San Lorenzo could not afford his wages or the transfer fee Newcastle would want, as he still had four years left on his contract. After lengthy talks with club officials and manager Alan Pardew, Coloccini decided to stay at St James' Park until the end of the 2012–13 season and then decide on where his future lies. On 16 June 2013, Coloccini confirmed that he was staying at Newcastle until the end of the 2013–14 season at least, after lengthy talks with Pardew. Coloccini also praised Newcastle as a club, saying that they had done everything possible to make him stay. On 1 November 2014, Coloccini made his 200th league appearance for Newcastle, in the 1-0 win over Liverpool at St James' Park. Eight days later, he scored his first goal in three years, with a header in Newcastle's 2–0 away win over West Bromwich Albion.
In July 2015, Coloccini was linked with a move to Crystal Palace to link up with former manager Alan Pardew on a permanent deal. This was later publicly dismissed by Pardew, in a statement released on 5 August 2015.
On 6 August, Coloccini ended speculation about his future by penning a one-year contract extension that keeps him at the club until the end of the 2016/17 season, with the option of a further year beyond that.
On 5 July 2016, Coloccini mutually agreed with Newcastle that his contract would be terminated, with him signing for Primera División side San Lorenzo that same day.