Michael Phelps is Secretly Married

Michael Phelps tied the knot with Nicole Johnson months ago, but they kept it under wraps ... TMZ Sports has learned.

We've learned the couple has been married since June 13th. The ceremony went down in Paradise Valley, AZ, where they just moved. It was officiated by Peter Carlisle, Michael's agent and longtime friend.

Michael and Nicole got the marriage license June 9th and filed the Record of Marriage June 15th.

It's fascinating. During the Olympics, when everyone was reporting they were engaged, they had already been married for 2 months.

We did find a clue ... on June 13th -- their wedding day -- Nicole posted a pic of the 2 of them casually dressed with the baby in tow with the caption, "Such a memorable night with my lil fambam."

And get this ... Michael did a Facebook Live last week and when some fans suggested he was already married he flatly denied it.

Belated congrats!!!