The 23-year-old former Liverpool youngster left Bradford after the club agreed to settle his contract in March and ended the season playing for Dundee.
Threlfall, a former England Under-19 international, failed to break into the first-team at Anfield and spent time on loan with Hereford, Stockport and Northampton before moving to Valley Parade in 2010.
Shrimps manager Jim Bentley told his club's official website: "Robbie is a great lad with a good attitude and I am delighted to have him here.
"We wanted a left-back who is a good defender but can also get forward and I think he will fit into the side really well."
Threlfall added: "I know a couple of Morecambe players and, as soon as they started talking about the place, I knew it would be a great place to come to. It looks a great set-up and I wish the season was starting tomorrow."