The 4,274,300 sales place the record fifth on the list of biggest selling albums of all time in the UK, along with What's the Story Morning Glory by Oasis (fourth), Abba's Gold - Greatest Hits (third), SGT Pepper's Lonely Heart's Club Band - The Beatles (second) and Greatest Hits by Queen (first).
21 has sold an average of 20,000 copies per week over the past four weeks, the total number of sales is expected to increase when this week's chart sales data is published on Sunday.
Official Charts Company's managing director Martin Talbot said: "Thriller has long been recognised as one of the most iconic albums of all time - there is now no doubt that 21 can be spoken of in the same breath."
The album has spent 66 weeks on the Official Albums Chart, 23 of those at Number 1 and outsold Amy Winehouse's Back To Black to be named the biggest selling album of the 21st century.
23 year old Adele has won six Grammy Awards and Two Brit Awards since bursting on to the music scene in 2008 and is estimated to be worth £6 million.