The 'Yummy' singer and the '7 Rings' hitmaker - who share a manager in Scooter Braun - are set to make a big announcement at 6pm (BST) and 10am EST.
Overnight, Justin tweeted: ''Special announcement tomorrow at 10 am pst ... (sic)''
Ariana retweeted the post and added: ''see u there everybody''.
The pop stars last teamed up on a performance during Ariana's headline set at Coachella last year.
The pair duetted on Justin's 2015 hit 'Sorry' for what marked Justin's first live performance in two years, which means they could release a recorded version of the duet.
Ariana teased that new music is on the way after she posted a studio snap last month.
Meanwhile, the pair recently joined the likes of Billie Eilish in launching a set of cloth face masks to raise money for musicians affected by coronavirus.
The 'bad guy' hitmaker, the 'god is a woman' singer and the 'What Do You Mean?' star are just some of the stars who have designed their own face masks to support the charity Helping Musicians UK.
Alongside the products, they wrote: ''100 per cent of the net proceeds from the sale of this product will be donated to the musicians benevolent fund, working as Help Musicians UK, with a minimum donation per product sold of £8/€8/$8 ... Helping Musicians UK is an independent UK charity for professional musicians of all genres, from starting out through to retirement.''
Each mask, which costs £15, features the artist's name or an iconic symbol relating to the artist. Ariana's mask features a single teardrop, which is a nod towards her 'no tears left to cry' era, whilst Justin's mask features his name in an graffiti style lettering.
Billie's Blohsh logo takes front and centre on her mask, whilst The Weeknd also designed one with an XO on the front, adorned by a heart.
The Rolling Stones famous lick tongue logo has been made into a mask, whilst Yungblud's pink scrawly logo features on another.
For fans of Frank Sinatra, one of the masks has been designed with his surname on in cursive writing and a trilby hat by the logo.
The biggest names in music royalty.
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