West announced the pregnancy on stage during a show at Revel in Atlantic City on Sunday night.
“Stop the music and make noise for my baby mama,” the 35-year-old hip-hop star said, referring to Kardashian who was in the audience.
The 32-year-old reality star is almost 12 weeks pregnant.
Kardashian’s sister Khloé Kardashian Odom celebrated the couple on Twitter: “Keeping secrets is hard with so many family members! Especially when you are so freaking excited!!!!! LOVE is everything!!!!”
Her husband Lamar Odom added: “I’m excited for Kanye and my sister! There’s nothing like bringing life into this world. Let’s keep God’s blessings coming!”
Sister Kourtney Kardashian also posted a congratulatory message via Twitter: “Been wanting to shout from the rooftops with joy and now I can! Another angel to welcome to our family. Overwhelmed with excitement!”
The siblings’ mother/manager Kris Jenner proclaimed: “Oh BABY BABY BABY. I’m a happy girl!!!!!!!!! Wowza!”
West and Kardashian officially started dating around March of 2012.
The couple apparently had a fling in 2010 while West was romantically involved with model Amber Rose and Kardashian was dating NFL star Reggie Bush. Their affair caused both relationships to end, Rose actually revealed during an appearance on the nationally syndicated “Wendy Williams Show” in 2011.
Kardashian is still fighting to legally end her 72-day marriage to NBA star Kris Humphries, which ended in October 2011.
VIDEO: Watch video of Kanye West stopping to acknowledge Kim Kardashian’s pregnancy.
PHOTOS: Check out the Twitter messages congratulating the couple.