The person spoke to The Associated Press on condition of anonymity Saturday because the deal had not been announced.
Rantanen will count $9.25 million against the salary cap. He's one of the final restricted free agents to sign a contract.
The 22-year-old Finn set career highs with 31 goals, 56 assists and 87 points last season. Rantanen has 209 points in 239 NHL games.