This is one birthday Justin Timberlake won't soon forget.

Jessica Biel, 26, threw her singer beau a Super Bowl-themed surprise birthday party in the Hollywood Roosevelt Hotel's ultra-exclusive penthouse on Sunday afternoon, PEOPLE has learned.

"Justin was totally blown away when he walked in. He had no idea," says a source.

On hand were close to 100 friends and family of the couple, including both of their parents, Biel's brothers, Timberlake's cousins and his best friend Trace Ayala, who joined the happy pair to toast Timberlake turning 28.

The penthouse was transformed with flat-screen televisions brought in especially for the festivities, and the party feasted on gourmet buffalo wings, pizza, mini sliders and red velvet cupcakes.

Biel planned the party down to every last detail, and visited the hotel to make sure everything would be just perfect, says the source.

And she was the perfect hostess. Wearing a little black cotton dress and flats, Biel worked the room while taking breaks to sit with Timberlake.

"They held hands and relaxed," says a guest. "They seemed very happy together."

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