The former Canberra Raiders stalwart, 28, has agreed to a two-year deal, and is expected to begin training on Monday.
He will be available for selection in the Cowboys’ Round 1 team to take on his former club Canberra at 1300SMILES Stadium on Saturday 4 March.
Of the 139-gamer, Cowboys General Manager – Football Peter Parr said the club was getting a quality, proven player.
“He’s a very good player and also very experienced,” Parr said of Fensom.
“He has come to us late in the piece and we were well-prepared to go with the squad we had put in place this season.
“But we were clear that if an opportunity arose to sign a quality player, then we would pursue that, and Shaun Fensom is a quality player. He plays in a position where we think we need to add some depth.”
New South Welshman Fensom is a two-time Raiders Player of the Year and had been with the Canberra-based club for eight seasons, debuting in 2009.