Rainier Joseph Diaz Castillo, better known simply as Rainier Castillo (born October 21, 1985 in Quezon City, Philippines), is a Filipino actor. He is known for his "killer smile" and identified with his F4 hairstyle of Jerry Yan. He made it as one of the Final Four in the first season of StarStruck, though he eventually lost the title of Ultimate StarStruck Male Survivor to Mark Herras. He can be seen in several shows in the GMA Network.

A freshman from AMA Computer University, he joined the reality based artista search "StarStruck" and was linked over 4 female survivor during the competition they are Jade Lopez, Yasmien Kurdi, Jennylyn Mercado and Sheena Halili. He was linked with Jade Lopez because during the competition Lopez had a crush on him, he was linked with Sheena Halili and was his loveteam on the competition but when Halili got eliminated their love team had stopped after the contest they got the biggest rumor about them that they had a relationship.

Though lost in the competition, StarStruck was his turning point for his career. This was considered the biggest thing happened to his life that provided door for more opportunities. He is visible in shows of the station such as teen-oriented shows like Click and Joyride, fantaseryes like Mulawin, Majika, Kamandag, Fantastic Man and Tasya Fantasya, comedy shows like Hokus Pokus and drama series like Maynila, Love to Love and Bakekang.

In 2009 he is choosen again to be pair to Yasmien Kurdi on the new installment of Sine Novela Dapat Ka Bang Mahalin.

Rainier has also a few commercials to his credit, such as Hapee Toothpaste (one commercial as an individual and another with Angel Locsin). He also was a model for Bench FIX Salon and was in Bench's 2004 Understatement underwear fashion show in Araneta Colliseum

• I Love You Babe

• Rainier Castillo at PogiSpotting
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• Rainier Castillo Message Board

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