Treat You Better Shawn Mendes
Little Mix and Taylor Swift were among the big winners at the ceremony, while Sam Smith, The Saturdays, Foxes and Ella Henderson were also in attendance.
Leigh-Anne Pinnock, Jesy Nelson, Perrie Edwards and Jade Thirlwall won Best Band at the event.
Lambert presented the group with their Best Band prize.
The singer turned to reveal a loose-knotted train on her outfit.
The ceremony took place in London's Berkley Square Gardens.
Emma won the gong for Best Presenter.