The 25-year-old striker, currently in Örgryte, has signed a three-year contract with the Allsvenskan club, an agreement that will start on 1 January.
"He is a moving and hard-working player who can play in all four offensive positions and will fit well into the Hammarby of the future," sports manager Jesper Jansson told the club's website.
Gustav Ludwigson has scored nine goals and six assists in 17 games so far this season, which is the most score of all players in the super league.