He told ABC he contacted the Powoshiek County Sheriff's Office to report the sighting after he heard news reports about a missing runner.
Riley said he was questioned by investigators and they searched his home.
Tibbetts, a college sophomore, was reported missing the next day, after she didn’t show up for her job at a local day care.
Before she disappeared, Tibbetts was living with her boyfriend, Dalton Jack.
His brother, Blake Jack, told Fox News on Wednesday there was no sign of a struggle at their home.
"She's small, but she would have done something," Blake Jack said.
Tibbetts' boyfriend has been ruled out as a suspect in the case.
Meanwhile, a newly released video of Tibbetts on the day before her disappearance shows her in a good mood, laughing and giggling.
The short clip was shared by her fellow camp counselor, Jarrett Rose, KCCI reported.
Robert Tibbetts, Mollie Tibbetts' father, told CBS he believes his daughter is with someone she knows.
"I think someone went to the house that Mollie knew or that Mollie trusted and that she left with them willingly," Robert Tibbetts said. "Now they're in over their head and they don't know what to do."
The reward for information on Tibbetts' disappearance has now exceeded $300,000.