Julio Arca has had his Middlesbrough contract cancelled by mutual consent and is returning to Argentina to recover from a toe injury.
The 32-year-old midfielder has made just one league and two cup appearances this season and had surgery on the problem during the campaign.
He returned to training briefly last month before further problems meant he was unable to regain full fitness.
"It's a shame for Julio and for the club," said manager Tony Mowbray.
Arca was Boro's longest-serving player and made 155 starts and 30 substitute appearances in all competitions, scoring nine goals.
He signed a new two-year contract at the end of the 2010-11 season after being voted the club's player of the year.
Arca was Gareth Southgate's first senior player when he signed from Sunderland in summer 2006 for £1.75m.