The Northern Irishman also urged the PGA and European Tours to join forces against the Saudi-backed venture on Wednesday.
Koepka, a former world No.1, has become the latest big name to join the lucrative Greg Norman-fronted LIV Series, joining a list that includes six-times major champion Phil Mickelson, former Masters winner Dustin Johnson and former US Open victor Bryson DeChambeau.
Koepka was the second player, behind Rory McIlroy, to speak out against a rival league with just 48-player fields in March 2020 when he said, "I have a hard time believing golf should be about just 48 players ... Money isn't going to change my life."
Among the PGA Tour's most vocal backers, McIlroy, speaking to the media ahead of this week's Travelers Championship in Connecticut, criticised Koepka and others for turning their backs on the PGA Tour after previously expressing their support.
"Am I surprised? Yes, because of what he (Koepka) said previously," said McIlroy, who has moved up to No.2 in the world rankings following a successful title defence at the Canadian Open and tie for fifth at the US Open.
"That's why I'm surprised at a lot of these guys because they say one thing and then they do another.
"It's pretty duplicitous on their part to say one thing and then do another thing."
The PGA Tour has suspended members who signed up to play with LIV Golf and said others who follow them will face the same punishment.
However, they were able to play at the US Open last week and were given the all-clear by the R&A on Wednesday to tee it up at the British Open at St Andrews from July 14-17.
The LIV Series will play its first American event in Portland, Oregon from June 30-July 2 at the Pumpkin Ridge Golf Club, going head-to-head with the PGA Tour's John Deere Classic.
Bankrolled by Saudi Arabia's Public Investment Fund (PIF), LIV Golf held its first event in London this month.
With more-and-more players tempted by the massive purses of the LIV Series, McIlroy called on the PGA and European Tours to come up a plan.
"I've always advocated for something where the tours work more closely together or we create some sort of world tour model or ATP model," said the four-times major winner.
"I would like to see the PGA Tour and the European Tour start to work more closely together and maybe try to forge a path to where all the biggest golf events in the world are under the same umbrella."
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