His office says Menendez popped the question to Alicia Mucci, his girlfriend of more than a year, Monday night.
Mucci lives in Bergen County and manages real estate investments.
Her public Facebook profile says she's widowed and worked as a "barmaid" at the Stony Hill Inn in Hackensack in August 2009.
The restaurant is home to numerous New Jersey political fundraisers and events.
Menendez's office says the couple met after a fundraiser but did not specify where or when.
The Democratic senator's first marriage ended in divorce in 2005.
