The 20-year-old Norwegian joins the Coyotes organization after spending the past three seasons primarily playing in the Eliteserien, the top professional league of his home country.
The left-shooting defenseman has racked up 94 regular-season appearances at the professional level in Norway, with three goals and 18 total points while playing for Sparta Sarpsborg - also commonly known as the Sparta Warriors.
He was named the top rookie in the Eliteserien after the 2018/19 season and has grown to establish himself as an international-level player.
Having captained Team Norway at the World Junior Championships (Division 1A) during the 2019/20 season, he earned a place with the national team at the World Championships earlier this year - featuring in six games and scoring a single goal in a 5-1 loss to Germany.
The Sarpsborg native is set to continue his development over in Europe, signed to a season with IK Oskarshamn of the Swedish Hockey League (SHL), allowing him to jump up to a higher level.
Already older than many other players in the draft, Lilleberg has the ideal size for the professional game, standing at 6-foot-2, 207lbs, and could potentially remain in Sweden for the next two seasons - unless he impresses the Coyotes organization enough to be called over to North America earlier.