The account @sussexroyal, set up on Tuesday, reached the one million milestone in just five hours and 45 minutes.
It now has more than 2.5 million followers and just one post so far – a gallery of photos of expectant parents Harry and Meghan, as well as people they met on their engagements.
The post has about 888,000 likes.
Guinness World Records said the previous record was held by K-Pop star Kang Daniel, who achieved one million followers in 11 hours 36 minutes after posting a selfie with the caption “HELLO”.
Before this, the record holder was Pope Francis, who reached one million in 12 hours after debuting his verified Vatican media channel franciscus in 2016.
Harry and Meghan’s new account comes as they split their household from that of the Duke and Duchess of Cambridge.