The new agreement will see the club's one-time main sponsor join forces with Boro once again with the naming rights for the South Stand at the Riverside.
It's envisaged that the new partnership will see 888.com delivering a number of exclusive offers to Boro fans, enabling 888.com to build the brand's presence on Teesside and beyond.
Boro Chief Operating Officer Mark Ellis said: "We're delighted to welcome 888.com to join our ever-increasing list of respected partners.
"We are continuing to develop and enhance our partnership portfolio and are looking forward to working closely with 888.com in the coming weeks and months."
Itai Pazner SVP of B2C 888.com said: "We are very proud to be announcing our stand sponsorship deal with Middlesbrough Football Club, who have both a fantastic history and truly great support base.
"With our footprint already established in the UK betting market, this deal for 888sport.com allows us to further engage with our target audience via the turnstiles and through on-going TV coverage."