The couple lost their baby last week, and Lisa said she needed to come to terms with her miscarriage before making the announcement.
In a post titled 'Unfortunate Announcement', the 27-year-old wrote, "Jack and I lost our baby boy last week. Having a late term miscarriage is by far the hardest thing either of us have ever had to go through.
"We appreciate all of your prayers and ask for privacy and respect during this time."
The couple, who are parents to 16-month-old Pearl, are doing their best to remain positive.
"It is important to stay hopeful and optimistic through life's toughest moments," Lisa wrote. "We never know why things like this happen. All we can do is trust in God and know he has a plan for our lives."
Jack Osbourne with daughter Pearl
Before heading to bed that night, she tweeted, "Cuddling my amazing, sweet hubby & shutting my eyes. Thank you for all the love & support. It means the world."
Jack and Lisa, both 27, married in Hawaii in October 2012, and announced they were expecting their second child just one month ago.