The 19-year-old, who hails from Toronto and first found success on the video-sharing site Vine, took home the prizes for best artist, best song, best Canadian act and biggest fans during the ceremony at London's Wembley Arena. The former Fifth Harmony star, who picked up the Best Pop award, is eyeing a third week at the top, but close behind - just over 2,000 combined chart sales - at Number 2 is Anywhere from Rita Ora, who hosted the show as well as performing her latest track.
USA rap star Eminem opened the show with a performance of his new song "Walk on Water", with Skylar Grey standing in for Beyonce to sing the other part in the duet. Despite receiving six nominations - the most of any artist - Taylor Swift failed to win any awards.
Notable performances included local London grime star Stormzy, who took to the stage poking out of the sunroof of a police auto, and Travis Scott, who entered standing atop what looked like a floating hawk.
Eminem was also named best hip-hop artist, and proclaimed surprise.
Speaking to accept his award - which was presented by Prophets of Rage - he said: "I'm not sure how I got this because I haven't had an album in a few years but I got one coming. Once again, MTV, thank you". He said: "Thank you". We love you. God bless you.
Irish band U2 collected MTV's Global Icon gong, previously awarded to Queen, Whitney Houston and Eminem. "They challenge us, they inspire us".