Mo returns to the club after being a regular feature in the midfield during the second half of last season.

A January loan signing, Mo made 15 appearances and played a key role as we reached the Sky Bet Championship Play-Offs.

Mo scored one goal during his time on Teesside – a terrific strike in a vital 2-1 win at Derby towards the end of the campaign.

Tony Pulis has made no secret of his admiration for the player and is delighted to secure his signature for the duration of the current season.

The 25-year-old, who had attracted interest from a number of clubs, completed the formalities of his move on Thursday.