The 22-year-old actress replaces last year's No 1 champ Heidi Klum on the yearly list released by the internet security firm, reported Ace Showbiz.
"Searching for the latest Emma Watson pictures and downloads yields more than a 12.6 per cent chance of landing on a website that has tested positive for online threats, such as spyware, adware, spam, phishing, viruses and other malware," said McAfee.
The second place was bagged by Jessica Biel. The fiancee of Justin Timberlake occupied the first place in 2009 and sat at the fourth spot last year.
Rounding up the top 3 is a new addition to the list, "Hitch" star Eva Mendes.
Klum is out of the top-10 list, while 2010's most dangerous celebrity Cameron Diaz claims the eighth place, six spots lower than last year's position.
Others are Selena Gomez, Halle Berry, Megan Fox, Shakira, Salma Hayek and Sofia Vergara.