The longtime couple hit headlines on Wednesday when it was reported the pair had tied the knot in mid-June (2016), a month-and-a-half before Phelps extended his record-breaking medals streak at the Rio Olympics in Brazil, where Nicole was frequently referred to as his fiancee.
On Thursday, the decorated athlete came clean to his fans in a Facebook Live video and confirmed he had been "married for a while", claiming he wanted to keep it a secret "just because".
Phelps made no mention of their plans to host an official wedding, which took place over the weekend in Cabo San Lucas, reports Us Weekly.
The happy couple was surrounded by around 50 friends and family members, including five-month-old son Boomer, at the lavish nuptials at El Dorado Golf and Beach Club, and the bride shared the first snap from their big day on Instagram in the early hours of Sunday (October 30).
"Truly the happiest day of my life," she captioned a shot of the newlyweds walking up the aisle following the ceremony.
The new Mrs. Phelps wore a white Julie Vino lace gown with a thigh-high slit, while her groom opted for a grey suit with white shirt and light blue tie.
Alongside another snap on Instagram, the athlete wrote, "My best friend.... I love yo!!"