Steven Greenberg spoke to Good Morning America in a segment that aired on Friday morning.
He pushed back against accusations of sexual abuse against Kelly, which are detailed in the six-part Lifetime documentary Surviving R Kelly.
Asked whether Kelly married Aaliyah – whose full name was Aaliyah Haughton – when she was 15, Greenberg said: “He was married to her when she was 15.”
Greenberg then referenced Priscilla Presley, who married Elvis Presley when she was 21 years old after meeting him aged 14.
He then claimed that Kelly wasn’t aware of Aaliyah’s age.
“My understanding is that she did not claim to be 15 and, in order to get married, she had to lie about her age. [He had] no idea,” Greenberg said of Kelly.
Kelly is now 52 and was 12 years Aaliyah’s senior.
Surviving R Kelly has prompted several celebrities to publicly denounce Kelly. John Legend participated in the documentary.
He later wrote on Twitter: “To everyone telling me how courageous I am for appearing in the doc, it didn’t feel risky at all.
"I believe these women and don’t give a f*** about protecting a serial child rapist. Easy decision.”
Lady Gaga and the band Phoenix have both apologised for working with Kelly in the past.
Despite facing a number of sexual misconduct allegations over the past two decades, Kelly has never been convicted. Last year, he denied accusations of holding young women in an abusive cult.