Marmion came to Connacht via the IRFU Exiles system which helps Irish-qualified players overseas to make their way into professional rugby in Ireland.
He went on to start every game of the 2012/13 season and was the sole Connacht squad member and only player in the league to do so. Now with 36 caps to his name, Marmion has not missed a game for Connacht since his debut 14 months ago.
This season, he tops the westerners' try-scoring chats having crossed the line in matches against the Ospreys, Saracens and Leinster. His stand-out performances earned him the Connacht Player of the Month award for September.
Born in Brecon, Wales but with family hailing from Loughrea, County Galway, Marmion has been capped at Under-18, Under-19 and Under-20 levels by Ireland and was named this year in the Ireland squad for the summer tour of North America.
Connacht head coach Pat Lam said: "I'm delighted for Kieran and for Connacht Rugby that we have this new deal.
"Kieran is a fantastic player and a key member of our squad and it's a real boost that he sees his continued development and future with the province he started his professional career with.
"From day one, Kieran has impressed me with his attitude, work ethic and incredible maturity and I have no doubt that he will go on to achieve great things within Connacht and Irish rugby."