Jason Zwick, 30, made the plea in in Miami County, just before jury selection was to begin. Common Pleas Judge Christopher Gee pronounced him guilty and sentenced him to 10 years to life in prison.
Prosecutors dropped two other charges that Zwick had raped the 10-year-old adopted son of the Troy man who has pleaded guilty to raping three boys. Zwick, of the Dayton suburb of Beavercreek, allegedly went to the adoptive father's home in Troy to rape the boy after the men communicated online.
The boy's 40-year-old adoptive father, who will serve 60 years to life in prison as part of his plea agreement, was expected to testify against Zwick, as was the boy, now 11.
Zwick's attorney had unsuccessfully sought Monday to delay the trial, alleging possible witness tampering. Attorney Joe Stadnicar wasn't available immediately for comment after the plea Tuesday.
Dressed in a dark suit, Zwick answered "yes" to a series of questions from the judge, who wanted to make sure Zwick understood his rights and the details of the plea and possible sentence. Zwick declined to make a statement before the judge ruled.
Zwick's plea means he doesn't contest the charge against him, without admitting guilt. Anthony Kendall, an assistant county prosecutor, said prosecutors expect him to appeal, but believe they are on solid ground for an appeal.
Gee also sentenced Zwick to pay a $20,000 fine and ordered him to regularly report his whereabouts as a sex offender to law enforcement if he becomes free. He can't live near a school or daycare.
"This is the most despicable thing I've ever seen," Kendall said after the plea hearing. "This is a guy out shopping for minors ... to have sex with. We got him off the street."
Gee had ruled earlier that the boy could testify via closed circuit television, instead of taking the witness stand in the open courtroom.
Zwick's attorney had asked Monday to delay the trial, charging that the adoptive father's mother had taken photos or video of Zwick during a jail visit, possibly to help her son or other witnesses identify Zwick in court. Stadnicar said in his argument Monday that the defense contended that Zwick had been wrongly identified as the culprit. Gee denied the motion for a continuance.
The Associated Press isn't naming the adoptive father in order to protect the children's identities.
The Troy man was a longtime foster father who had adopted two boys and a girl and was in the process of adopting a fourth child, another boy, when arrested last February. The children were ages 9 to 12 at the time of his arrest.
He also has pleaded guilty in neighboring Montgomery County to child rape and complicity to rape. In that case, he was charged with taking the same boy as in the Zwick case to the home of Patrick Rieder, 32, of Dayton. Rieder's trial is scheduled Feb. 19 on four counts of rape of a child under 13.
The adoptive father also agreed to testify in Rieder's trial. His Montgomery County sentencing is tentatively set for Feb. 26, and the plea agreement there calls for 50 years to life in prison.
Montgomery County Prosecutor Mat Heck Jr. last month called the adoptive father a child abuser who "will basically spend the rest of his natural life locked up."
The adoptive father told The Associated Press in a recent interview that he was sorry for hurting the children, and hoped they would be spared from testifying. The children were placed in the care of the county children's service agency after his arrest.
Authorities say they made the arrests after an investigation that began with an undercover detective checking into an online posting about "taboo sex."