Newark Mayor Cory Booker filed the paperwork on Tuesday.
It comes as no surprise. The Democrat announced last year that he would not run for New Jersey governor this year but instead was looking at the Senate in 2014.
The seat is currently held by 88-year-old Democrat Frank Lautenberg. Lautenberg has not indicated any plans to retire from the Senate.
When Booker expressed interest in the seat last month, Lautenberg's spokesman said the senator would consider his options later.
U.S. Rep. Frank Pallone has also said he's interested in running.