"Rob and I are so excited it's stupid," Olson says in a statement. "The last couple years have been pretty amazing for us and just when we thought things couldn't get any better, the greatest little thing in the world happened! We're totally beside ourselves."
Olson, 34, and McElhenney, 32, married in Sept. 2008 in Malibu, Calif.
FX cult hit It's Always Sunny, which McElhenney created and executive produces, will return for its sixth season in September. Olson most recently appeared in Leap Year as the best friend of Amy Adams.
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Reporting by ELIZABETH LEONARD