Billie Eilish Dances Above Adoring Crowd Outside Radio City Music Hall

Billie Eilish gave an encore performance in NYC ... from her window, and her fans roared in approval.

The 17-year-old alt-pop singer rocked back and forth Wednesday night from her dressing room at Radio City Music Hall as a huge crowd gathered below ... blasting Billie's hit after hit. The "Ocean Eyes" singer had just performed there and the crowd clearly thirst for more. And, she gave it to them.

Watch ... Billie danced and recorded her fans down below. She posted her vantage point on Instagram and you can see why she's cemented her status as a bonafide star. BTW ... her gig at the iconic Radio City Music Hall followed Tuesday night's NYC debut at Rooftop at Pier 17 ... which sold out.