The 'Rolling In The Deep' hitmaker's second album - which was released in January 2011 - has sold 4,274,300 copies in Britain, more than 500 more than the late singer's 1982 classic record, meaning it is now the fifth best-selling album of all time in the UK.
Official Charts Company's managing director Martin Talbot said: ''To add to her many landmark records, Adele's achievement in overtaking 'Thriller' in the all-time sales list is truly remarkable.
'''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.''
According to the organisation, '21' - which has spent 23 weeks at the top of the UK singles chart - continues to sell around 20,000 copies a week.
This means it could catch the four other albums which have currently outsold it in the UK.
Topping the best-selling list is Queen's 'Greatest Hits' with 5.83 million sales, followed by The Beatles' 'Sgt Pepper's Lonely Hearts Club Band'.
To date, 'Thriller' has sold more than 100 million copies worldwide while '21' has clocked up global sales of around 20 million.