"Welcome little baby boy Adler!!!!" Rachel wrote earlier this week.
"Our family of 3 has become a happy family of 4!!!!! We couldn't be more excited. This guy has filled our hearts and home with so much joy. Thankful for my hubby @brendonvbb12 who is the best partner and such a strong rock in our family!!!"
Rachel added, "Thankful for my daughter @adoravillegas who is mommy's helper and best friend."
Adler joins his big sister Adora Borealis, who was born in 2016 and turned four years old this past April.
Comments of support and well-wishes poured in from Big Brother alums, including Jessica Graf, who wrote, "I love it!! Love you guys so much! Glad everyone is healthy."
Rachel toldUs Weekly that her son's middle name is Mateo, so her boy's full name is Adler Mateo Villegas.
When Brendon, who competed on Big Brother and raced around the world with Rachel on two seasons of The Amazing Race, announced on Twitter that Rachel had given birth, he said mom and baby were "healthy and doing well" but he and Rachel had yet to decide on a name.
Rachel revealed to Us that she actually had a "super hard and scary" labor and delivery.
"At one point I was on oxygen and it lasted longer than my birth with Adora," Rachel explained.
"Almost 15 hours he did not want to come out of the womb. Brendon was really supportive and my rock the whole time. He encouraged my breathing and my labor positions and helped me when I didn't think I could keep going. He was an amazing partner and has been so helpful."
Rachel gushed about how her daughter has been "so helpful" and "very protective" of her little brother.
While Rachel admitted she and Brendon haven't been "sleeping at all" with their newborn at home, the parents couldn't be happier.
"Our little Adler is such an amazing addition to our family," Rachel told the magazine. "We can't wait to get to know what his personality is going to be like. Our hearts couldn't be more overjoyed and full."
In addition to competing on Big Brother and racing with Brendon twice on The Amazing Race, Rachel also competed on The Amazing Race's 31st season with her sister Elissa Slater.
"I'm already planning their reality TV appearances," Rachel joked of her kids in a recent interview with People.
Rachel and Brendon first met when they competed on Big Brother's twelfth season in 2010.
After beginning a "showmance" during the competition, the couple continued dating and got engaged on Valentine's Day before returning to Big Brother in Summer 2011 as part of the thirteenth season's "duos" twist in which three pairs of former houseguests were given a second chance to compete on the show.
While Brendon was evicted -- twice -- from theBig Brother house midway through the competition, Rachel made it to the Season 13 finale, where she took home the show's $500,000 grand prize after winning a 4-3 jury vote over fellow contestant Porsche Briggs.
The couple then made their third appearance on a CBS reality series when they competed on The Amazing Race's 20th edition in early 2012. They finished the season's final leg in third place.
After their Amazing Race stint, Rachel and Brendon got married on television atop the AT&T Center, a 32-story high-rise in downtown Los Angeles, in September 2012.
Rachel reportedly arrived at the wedding in a silver-sequined Missoni gown at sunset via helicopter.