Koepka, whose brother Chase played in the opening event at Centurion Club earlier this month, is expected to play in the second 54-hole, 48-man tournament - which has a prize fund of $25m - in Portland, Oregon, next week.
The former world number one will be suspended by the PGA Tour as a result of his decision, which comes after he complained that questions on the topic at Brookline were throwing a "black cloud" over the US Open.
Asked in his pre-tournament press conference why he had seemingly decided to stay on the PGA Tour, Koepka said: "There's been no other option to this point, so where else are you going to go?"
When the reporter pointed out that the LIV events had started, Koepka added: "As of last week. That's it. I wasn't playing last week.
"I'm here. I'm here at the US Open. I'm ready to play US Open and I think it kind of sucks, too, you are all throwing this black cloud over the US Open. It's one of my favourite events. I don't know why you guys keep doing that.
"The more legs you give it, the more you keep talking about it."
Koepka won the US Open in 2017 and 2018 and the US PGA Championship in 2018 and 2019, but the 32-year-old has been hampered by injuries in recent years and is currently ranked 19th in the world.
Nevertheless, his defection will be a big blow to the PGA Tour on the day that commissioner Jay Monahan had called a player meeting at the Travelers Championship to provide updates and answer questions.
Monahan suspended 17 players as soon as they teed off in the inaugural LIV Golf event, having been refused the required releases, and said the "same fate holds true" for future rebels.
Patrick Reed, Pat Perez and Bryson DeChambeau joined LIV Golf during that first event, the latter famously having a feud with Koepka which culminated in them playing a 12-hole match, won easily by Koepka, in Las Vegas in November.
LIV Golf is expected to announce more signings soon and CEO Greg Norman believes it has a "compelling" case to be allowed to award world ranking points in future events, which would be a significant plus.