The 23-year old, who appeared just once for the Blues, spent two separate spells at Oakwell, impressing during a short-term deal in 2011 before signing permanently in 2012.
The English midfielder was highly rated as a youngster, joining Chelsea as a 16-year-old for £1 million, before reaching the final of the FA Youth Cup in 2008, and earning national caps for the England Under-16 to Under-19 age groups.
Mellis' one appearance in a Chelsea shirt came in 2010, when he was an injury-time substitute in a Champions League group-stage victory over MSK Zilina.
His first spell with Barnsley came in 2011, when he scored two goals in 15 matches, before returning to Stamford Bridge.
However, Mellis' future at Chelsea was cut short in March 2012, when he was sacked by the club after admitting to setting off a smoke grenade which caused a full-scale evacuation of the training ground in Cobham. Three months later, he had returned to professional football, signing a two-year contract with the Tykes.
However, Barnsley have announced that the player has left the club following the official opening of the summer transfer window.
In a statement released on the official club website, officials revealed that Mellis was not willing to sign a new contract with the team currently in League One, believing that he needed a fresh challenge to revive his career.
The statement read: "Club officials met with Jacob Mellis and his parents on Friday 27 June. During the meeting it became clear that Jacob felt he needed a fresh challenge and that he would prefer to leave the club than be part of the club’s League One campaign.
"The club have therefore decided to withdraw the offer of a new contract to Jacob and he will now pursue opportunities away from Oakwell.
"Jacob’s decision is disappointing after the opportunity that the club afforded him over the last two seasons. However, the club are only interested in having players completely committed to ensuring next season is as successful as possible and the club ultimately return to the Championship."
It is likely that Mellis will have a number of suitors in the Championship, as his two-year spell with Barnsley saw a run of impressive performances, and any club looking to acquire his services will be able to do so for free.