Michael Phelps, Nicole Johnson secretly married in June

Michael Phelps can add getting married to his long list of accomplishments this year.

According to documents obtained by multiple outlets, the 31-year-old Olympic swimming champion secretly married longtime girlfriend Nicole Johnson on June 13, a little more than a month after the former Miss California USA gave birth to their son, Boomer.

The couple announced their engagement in February 2015. Phelps' marriage was first reported by TMZ.

A copy of a marriage license shows Phelps and Johnson were married in Paradise Valley, Arizona. Neither Phelps nor Johnson said anything about the wedding. Johnson posted a picture of her with Phelps and Boomer on Instagram on the day of the ceremony with the caption, "Such a memorable night with my lil fambam."

Phelps, who announced his retirement after winning five more gold medals in this year's Rio Games to increase his overall haul to 23, said having his family there in Rio to cheer him on is a cherished memory.

"When I got out of the warm-up pool for the last time to get ready to go up to the blocks, I have the white shoes that I wore pretty much every finals," Phelps told ABC News. "In the left foot, there's a footprint of him, inside of my shoe, and I put my shoe on and I looked down at it and I started to get emotional because I knew he was in the stands.

"Being able to have our first child witness my last Olympics ever, my last ever, I mean, it's just dream come true after dream come true."

The Associated Press contributed to this report.