The 25-year-old pop superstar - who was videoed hugging Billie Eilish during Ariana's set last weekend - broke his hiatus to duet with the 'Thank U, Next' hitmaker on his 2015 hit 'Sorry'.
The performance was a complete surprise as Ariana, also 25, had said there would be no special guests joining her for her second performance at the festival.
During his impromptu appearance, which came after Ariana sung 'Break Free', Justin - who is married to Hailey Bieber - told the California crowd: ''I haven't been on stage in like two years. I came out here, no idea I was gonna be on stage tonight.
''I wore this fly outfit not knowing I was gonna be on stage. I had to get my groove back, get my swag back.''
The 'Love Yourself' singer - who shares manager Scooter Braun with the 'No Tears Left to Cry' hitmaker - then said: ''Thank you Coachella, thank you to Ariana Grande. She's so amazing by the way, she's so awesome.''
If that wasn't enough, Justin then teased that his new album - his first since 2015's 'Purpose' - is ''coming soon''.
Justin - who recently made his music return on Lil Dicky's song 'Earth' with Ariana, and the likes of Ed Sheeran, DJ Khaled, Katy Perry, Halsey, Shawn Mendes and Adam Levine - told fans in March that he wouldn't be rushing his return to music because he wanted to take the time to make something he could be proud of.
He wrote in a lengthy social media post: ''So I read a lot of messages saying you want an album .. I've toured my whole teenage life, and early 20s, I realised and as you guys probably saw I was unhappy last tour and I don't deserve that and you don't deserve that, you pay money to come and have a lively energetic fun light concert and I was unable emotionally to give you that near the end of the tour.
''I will come with a kick ass album ASAP, my swag is undeniable and my drive is indescribable his love is supernatural his grace is that reliable.... the top is where I reside period whether I make music or not the king said so. but I will come with a vengeance believe that.. (grammar and punctuation will be terrible pretend it's a text where u just don't care). (sic)''
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