The photo, with Upton in red trousers, white tank top and red jacket, was showing just a hint of a baby bump.
The couple's fans started sharing their congrats on her post right away.
"God Bless you both at this wonderful time", another wrote.
Verlander reposted the same picture of his wife on his Instagram with a loving message.
"You're going to be the most unbelievable Mum", he wrote.
It is unknown as to whether or not Justin is in Miami with her as she breaks the news. They started dating in 2014 and got engaged in 2016 but they didn't share their wedding date with the public until it happened. I love you so much.
"You know, I work really hard on myself: on feeling good, working out, being strong", she said about her top spot.
Kate appeared on the cover of Sports Illustrated's swimsuit issue in 2012, 2013, and 2017. Kate was on hand for Verlander's World Series win when the pitcher helped propel his Astros to victory.