"So thankful to have met my better half 9 years ago today!" the swimming champ wrote. "I love you so much @nicole.m.johnson .... So happy to be able to call you my best friend! And I'm looking forward to the future and watching our amazing healthy little boy @boomerrphelps grow!!"
Johnson penned her own sweet message to Phelps on her Instagram page, writing, "9 years ago today you walked into my life @m_phelps00 who knew through all the ups and downs that 9 years later we would have started to build our family ? I love you and I can't wait for what our future holds!!"