Jason Zwick of Beavercreek must also pay a $20,000 fine, the Dayton Daily News reported. He has been classified as a Tier III sex offender.
Zwick entered the plea as his trial was scheduled to begin.
First Assistant Miami County Prosecutor Anthony Kendell said the sentence is the same one Zwick would likely have received had he been convicted at trial. He said the three counts of felony rape Zwick was charged with stemmed from a single act and would have been merged for sentencing.
"I'm pleased that we got the life sentence, the maximum sentence from our perspective," Kendell said. "This is a guy that's out shopping for minors to have sex with. We're talking 8, 9, 10 years old."
The victim's adoptive father, Kenneth H. Brandt, 40, was sentenced to 60 years in prison in Miami County and is expected to receive another 50-year sentence in Montgomery County under a plea agreement. That sentencing is scheduled for February.
Brandt molested his adoptive sons and peddled them to other pedophiles, investigators say.
"I've been doing this for ... 20 years this year and this is the most despicable thing that I've ever had to deal with and I've had to deal with a lot of ugly cases," Kendell said.