The 39-year-old singer said the R&B star and Sinatra share a similar "thick tone," reported Femalefirst.
"She is the Frank Sinatra of our generation. She can turn anything into gold with that voice...
"Rihanna has a thick tone, so it's very hard to annoy anybody. It's like a beautifully squeezed tube of toothpaste. When you think of Rihanna's voice you think of this whole, rich thing, solid like a tree trunk," he said.
He also compared "Hotline Bling" hitmaker Drake's voice to Rihanna, 28, and Sinatra's.