Striker Magaye Gueye has signed a one-year contract with Millwall, with the option of a further 12 months.
Gueye, 24, was a free agent after leaving Everton in June. He made 39 appearances for the Premier League side, scoring three goals.
He also represented Senegal at the 2012 Olympics, helping them to reach the quarter-finals.
Gueye has been training with the Lions for the past couple of weeks and featured in Millwall's pre-season friendly at Dartford.
He becomes Ian Holloway's fifth signing of the summer following the arrivals of Lee Gregory, Ricardo Fuller, Carlos Edwards and Byron Webster.