Boro signed experienced Greek goalkeeper Dimitrios in August 2013 on a short-term contract until January 2014, which was later extended to the end of the season.
However, his impressive performances between the sticks led to him being awarded a new one-year contract in May 2014.
Known as "Dimi", he made 117 appearances for neighbours Hartlepool from 2003 to 2007 before moving to Coventry, where he played 31 games during a four-year spell.
He started his career with Greek side Kalamata in 1996 and also played for Boro's UEFA Cup opponents Egaleo before moving to England.
He has also played on loan at Swansea and Cardiff and has just finished a two-year spell at AEK Athens, where he made 39 appearances.
Capped once at senior international level for Greece, he also won nine U21 caps.
He made his Boro first team debut in the 2-0 FA Cup with Budweiser defeat to Hull City, ending the season as Aitor Karanka's first choice.
Dimi was part ofthe record-breaking Boro rearguard who racked up nine consecutive league clean sheets in the 2015/16 season, smashing a 30-year club milestone. The Greek stopper kept an incredible 22 clean sheets throughout the campaign on Boro's road to promotion.