According to TMZ, McGregor didn't like that the fan was trying to take a photo of him outside of the hotel.
McGregor, 30, was booked and charged with strong-armed robbery and criminal mischief, police confirmed to Fox News.
He then picked up the phone - worth $1,000 (£755) - and walked away with it.
Conor McGregor's mugshot, via Miami Police.
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McGregor's lawyer Samuel Rabin told The Herald in a statement that his client "was involved in a minor altercation involving a cell phone that resulted in a call to law enforcement".
The entire incident was apparently captured on surveillance footage, which eventually led detectives to McGregor where he was arrested for the crime.
McGregor previously entered a plea deal in Brooklyn last summer stemming from an incident at Barclays Center on April 5, when he was caught on video throwing a metal dolly into the window of a bus stationed inside the loading dock.
Check below for the top Twitter reactions to McGregor's arrest.
Mixed Martial Arts star fighter Conor McGregor was arrested Monday evening after police said he smashed a fan's phone outside the Fontainebleau Miami Beach hotel.