The 29-year-old singer has been married to hip-hop mogul Jay-Z since 2008.
Beyonce's pregnancy was obvious beneath her orange evening gown at Sunday's MTV Video Music Awards in Los Angeles.
"I have a surprise!" she told photographers snapping her picture.
She performed "Love on Top" during the show, urging the audience "to stand up on your feet."
"I want you to feel the love that's growing inside me," she said, rubbing her belly as she wrapped up the number.
"They told friends in the last month," a source told People.com. "She's never been happier. Seriously, it's all she's been waiting for. And Jay is like an excited kid. They're adorable."
Beyonce did not say when she is due to deliver or what the baby's gender is.