The multiple Tour de France winner's girlfriend, Anna Hansen, has given birth to the couple's first child. They have named their new son, Max.
Armstrong twittered the good news to fans.
He wrote on Twitter: "Wassup,world? My name is Max Armstrong and I just arrived. My mommy is healthy and so am I!"
Armstrong added that baby Max weighed in at 7lbs 5oz and is 20 inches long.
Hours after he finished 12th in the Giro d'Italia grand tour in Rome on Sunday, the cyclist posted on Twitter that he was on his way to Colorado to welcome his fourth child.
"Already at the airport in Rome," Armstrong posted on Twitter. "Headed home!! Baby #4 any minute now. CaNot wait!"
Armstrong, a cancer survivor, told Usmagazine.com in December that he and Hansen were expecting.
"Anna and I are thrilled to confirm that we are expecting in June and our families are ecstatic and grateful," Armstrong told Us at the time. "We are very much looking forward to what 2009 brings on many fronts. We appreciate [the public] respecting our privacy, as we are both eager to celebrate the holidays as a family."
Armstrong, 37, has three children from a previous marriage. He is father to son Luke, 9, and 11-year-old twins Isabelle and Grace with his ex-wife Kristin Richards. They had the three children using in vitro fertilization with sperm he had banked before undergoing chemotherapy.
Armstrong's baby with Hansen -- whom he has been dating since July after meeting through his charity work -- "was conceived naturally," a Lance Armstrong Foundation spokeswoman said in a statement to the Associated Press.