The 30-year-old actress made the announcement on Tuesday's Jimmy Kimmel Live, when she admitted that she was hesitant to wear her tight-fitting dress because she's "carrying a little extra weight."
"You're either pregnant or have a beer belly," Kimmel joked, which prompted Reeser to announce that she was expecting.
Reeser, who has also appeared on Entourage and The O.C., married writer-director Jesse Warren in 2009. This is couple's first child.
Watch the clip below: