The 38-year-old actress - who announced she and her Hollywood agent husband David Bugliari were expecting their first child last month - has admitted she is "really bad at keeping secrets" and so has decided to reveal the sex of their child to the world.
Writing on her official website, Alyssa said: "As fun as it was reading all of your gender predictions on twitter, I`m really bad at keeping secrets and had to share (not to mention, I was bound to unintentionally slip at some point).
"David and I are elated to announce that we are having a baby boy. We can`t wait to meet him. Boys names, anyone?"
The brunette beauty - who has previously dated singer Justin Timberlake and late actor Corey Haim - recently explained she was "overwhelmed" at the support she has received from well-wishers over the course of her pregnancy.
She tweeted: "I am overwhelmed by the outpouring of well wishes! I`m reading each one w/ big chunky tears in my eyes. GROUPHUG Get in. (sic)"
The `Charmed` actress also confessed she is suffering from really "odd" cravings, including a new-found addiction to broccoli.
Alyssa said: "I can`t even consider it a craving because it`s so odd. But I can`t get enough broccoli!"