The model announced the happy news via Facebook on Tuesday, writing, "Well, the day just arrived and we couldn't be more happy!!! Noah Phoenix was born yesterday afternoon and already stole our hearts. Anja is a proud big sister now and we are so thankful he is with us."
"Welcome my son, can't wait to show u around this crazy place," fiance Jamie Mazur tweeted on Monday.
The couple reportedly began dating in 2005. This is the second child for Alessandra and Jamie, as they welcomed a baby girl, Anja Louise, in 2008.