Kim, who won The Players Championship in 2017, has been known to wear PGA Tour Superstore shirts on the PGA Tour in recent times.
But this one was loud. Really loud.
The shirt features the words 'PGA Tour' all over it.
You can only wonder what Mickelson was thinking as he shook hands with four-time PGA Tour winner Kim on the first tee earlier today.
Then again, Mickelson, a 45-time PGA Tour winner, is wearing his LIV Golf 'Hy Flyers' gear at The Masters this week so it's probably fair game!
Kim may have gone 1 up in the shirt wars, but Mickelson certainly got the better hand on the course as he carded a solid 69 to Kim's 72.
Mickelson looks on from 4-under par in the tournament and eight shots off the pace set by Brooks Koepka, while Kim is at 1-over par.
Mickelson of course became the first high-profile player to move from the PGA Tour to LIV Golf last summer.
Lefty reportedly signed to LIV Golf in a three-year deal worth $200m.
Here's a look at how golf social media reacted to Kim's choice of shirt:
Can only really sit back and admire Si Woo Kim's choice of threads while playing with Phil Mickelson today pic.twitter.com/sIsbEPJh6D— Alex Perry (@AlexPerryNCG) April 7, 2023
Playing with Phil, Si Woo is wearing a shirt with PGA tour written all over it. pic.twitter.com/DfzZRcWTEj— RORY FANS (@rory_fans) April 7, 2023