The 26-year-old, who may be deployed in as an attacking midfield or a forward, has played in the Greek Superleague since 2012 with Xanthi and last turned out for Apollon Smyrnis from the gone year.
Prior to plying his trade in Greece, Triadis represented the SV Wehen Wiesbaden II and SV Wehen Wiesbaden sides in Germany.
The Greek-origin German has represented some of the German youth national teams ranging from U-15 to U-19 age groups.
Triadis is replacing Ghanian midfielder Augustine Okrah who made only four appearances this season before sustaining an injury that has kept him out of action since.
NorthEast United are currently among the top four teams in the regular season with 20 points from 12 matches and are set to resume their 2018-19 ISL campaign with a clash against Chennaiyin FC on Saturday, January 26.