The couple already have a son named Camden Jack Cutler, reported Digital Spy.
"We decided to announce that.... We're having another boy!! We couldn't be more excited!," Cavallari tweeted.
The couple welcomed Camden in August 2012 and wed last year.
Cavallari, 27, became famous after starring in MTV's reality series 'Laguna Beach', while Cutler is the quarterback for the NFL's Chicago Bears.