Simmons' agent, Rich Paul, had told ESPN's Adrian Wojnarowski last week that his client was unlikely to play in the event.
"I will still be heading back home to Australia to host my camps as well as train and play with the Boomers in the upcoming Exhibition Games," Simmons wrote on Twitter, referring to exhibition games against the United States in Melbourne next month. "I'm really excited about the talent we have on the Boomers squad, especially moving closer to 2020 where I will be honored and humbled to represent my country on the world's biggest sporting stage at the Olympics in Tokyo.
"Ultimately, we decided it was best that I used the time in September to return to Philadelphia to acquaint myself with my new teammates and prepare for the upcoming NBA season."
Simmons debuted for the Australian national team as a teenager and won gold at the 2013 FIBA Oceania Championship. He opted out of the 2016 Olympic squad to focus on his rookie season with the Sixers.
After Paul's comments to ESPN, Australia coach Andrej Lemanis said he was in the dark about Simmons' decision.
"The reality of the professional basketball environment is that being available to play for the national team is fluid for all our players," Lemanis said. "There are many elements under consideration here."
Also on Tuesday, Simmons' five-year extension with Philadelphia through the 2024-25 season was officially announced by the team. The Sixers did not disclose financial terms, but sources told Wojnarowski that he received the maximum $170 million extension.
"I want to say thank you to everyone who reached out to congratulate me on the extension with the 76ers," Simmons wrote Tuesday. "Philly means so much to me, and I'm already excited for next season."