The Wipeout host shared the news on her official website, after announcing in March that she is pregnant with her and husband Nick Lachey's first child.
Vanessa wrote: "It's a BOY. Nick and I are incredibly excited to share the news that we are expecting a baby boy. We can't wait to meet him soon!!!"
The 31-year-old had previously admitted that she "always wanted a boy first" in order for him to take care of his siblings, although she went on to add that a baby girl would be "just as much of a blessing".
Vanessa also revealed in April that "every day [of her pregnancy] has been an adventure", and that hormones are making her bad-tempered and quicker to snap at her spouse.
The couple's son is due in September.