By ALYSSA LITOFF and LAUREN EFFRON

Jason Derulo is so sure that his girlfriend Jordin Sparks is the one for him that he wrote a song about proposing to her aptly titled, "Marry Me," and had her star in the music video with him.

"That's the reason why I wrote the song in the first place," he told "Nightline," "because I wanted her to know that when the time is right then it will happen."

But don't think he'll necessarily be heading to the altar any time soon. The 24-year-old pop star said he wants to wait for the right moment before putting a ring on the former "American Idol" winner's finger.

"It will be the most amazing time," Derulo said. "When I do get down on one knee, it's going to be one of the most amazing experiences of my life, but when the time is right."

Derulo has sold more than 20 million singles in the United States, and has had three songs hit number one on Billboard's Mainstream Top 40. He launched his career writing songs for "everybody from Pitbull, to Danity Kane, to Cassie, to Sean Kingston, Iyaz," he said, before breaking out as a star on his own.

Derulo and Sparks have been an item for more than two years. They sing together in "Vertigo," a track from his latest album, "Talk Dirty."

Derulo said they work well as a couple because they are "polar opposites." The thing he loves most about her, he said, is that she reminds him of his mother.

"At this point, it's really scary," Derulo said. "My mother doesn't really have a filter. She's really a sweet, sweet woman. She'll tell you how it is… Jordin's the exact same way. She just doesn't have that filter."

He also said his girlfriend is "always thinking about others" and is "super charitable."

"She's opened me up to new things," he said.