The 65-year-old crooner has been dating Alexis, a former Morgan Stanley executive, since 2009 after he split from his ex-wife Katie Lee.
The pair confirmed their exciting news to People magazine via their representative.
A source has revealed that the baby, due mid-2015, is a girl.
โBilly is thrilled to become a dad again,โ the insider told People.
The Piano Man hitmaker already has one daughter, singer/songwriter Alexa Ray, 29, from his previous marriage to supermodel Christie Brinkley.
Now, almost three decades later, sheโll have a little sister!
Billyโs rep also states that the couple are opting โto keep any further details of her pregnancy under wraps until their new addition makes an official debut.โ
Billy and Alexis were initially friends before a romance blossomed.
The pair now live together in Oyster Bay, New York and reportedly have no immediate plans to tie the knot.