E! Online reports that Taylor hosted a dinner for her friend, born Ella Yelich-O'Connor, at ZZ's Clam Bar in Manhattan.
Celebrities on hand to celebrate included Lena Dunham, Karlie Kloss, Mae Whitman, Tavi Gevinson, Aziz Ansari and Lorde's childhood friends from New Zealand. Taylor decorated the group's area with gold balloons spelling out "U R 20." Lorde posted a photo of the setup, saying she was the "happiest birthday girl in the world."
The bash included a candle-festooned chocolate birthday cake, presented to her by Taylor.
After the party, Lorde posted a photo of herself in between Taylor and Karlie, who were kissing her cheeks. The caption read, "Had the best birthday party i've ever had tonight surrounded by my nyc family. all organized by tay who is, as she says, 'a mom with no kids'."
She added, "i am ur kid and u love me so hard i could burst. here's to our 3 magic years of best friendship and more moments like this, squished between angels."
Earlier in the day, Taylor posted a birthday card she'd painted for Lorde, with the caption, "Thank you for the music you make, the advice you give, the thoughts you provoke, and the way you've made my life more beautiful since the first day we met up in New York and ate burgers on a park bench and got attacked by squirrels. I. Love. You. So. Much. Ella."