The singer has enjoyed huge success with the record and is currently on course to remain at number one on the UK album chart for the 11th consecutive week.
Take That previously held the title of most downloaded album on Amazon with their 2008 album The Circus.
Roger Greensmith, music and MP3 manager at Amazon.co.uk, told the Sunday Mirror: "Adele's success, particularly following her Brit Awards performance in February, has been phenomenal.
"2011 is already shaping up to be Adele's year with 21 achieving so much and her previous album 19 is also flying high in the Amazon MP3 download charts as our second most-downloaded album of the year."
The top ten most downloaded albums of all time on Amazon.co.uk:
1. Adele - 21
2. Take That - The Circus
3. Kings of Leon - Only By The Night
4. Mumford & Sons - Sigh No More
5. Take That - Progress
6. Lady Gaga - The Fame Monster
7. The Killers - Day & Age
8. Lily Allen - It's Not Me, It's You
9. Florence & The Machine - Lungs
10. Plan B - The Defamation of Strickland Banks