Kim was sworn in by Vice President Kamala Harris on Monday.
He fills the seat previously held by fellow Democrat Bob Menedez, who resigned after being convicted of federal bribery charges.
Kim is the first Asian-American to win that post in New Jersey, and will be the first Korean-American to serve in the Senate.
Interim Sen. George Helmy, a Democrat appointed when Menendez resigned over the weekend so Kim could be sworn in early.
The move was expected and confirmed what Helmy and Democratic Gov. Phil Murphy announced during his appointment this summer - that Helmy would resign early so the winner of November's election could get into office before the start of the new session of Congress next month.
Murphy explained the arrangement, saying he wanted the "democratically chosen winner" of this year's election to have a smooth transition into office.
Kim defeated Republican businessman Curtis Bashaw on Election Day.