Clucas has left Dungannon Swifts after almost five years in the colours of his hometown club and was awarded the captain’s armband for the current campaign.
The Glens’ head coach, Mick McDermott, has praised the overall attributes of the 27-year-old.
“Seanan will be a great addition to our team,” said McDermott on the club’s official website. “He brings quality, a physical presence and can play in multiple positions.
“He is a proven quality performer in the Danske Bank Premiership.
“I am absolutely delighted to have him at Glentoran.
“I would also like to thank Dungannon Swifts for the professional way in which they have conducted this deal.”
Clucas also spent time on the books at Linfield and Derry City alongside spells across the water with Preston North End and Bristol Rovers.
The arrival of Clucas marks another strong statement of intent as Glentoran attempt a title drive across the second half of the season.
The Glens have also had bids rejected for Joe Gormley and Adam Lecky in this current transfer window.
Glentoran face Glenavon tonight at The Oval two points off league leaders Crusaders but with a game in hand.
Clucas will have to delay his Glentoran debut due to an ongoing suspension.