The movie website released the list, which is based on the number of views received by actors' profile pages, on Tuesday.
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"Pirates of the Caribbean" star, who is now No. 3, had held the No. 1 perch for six of the past seven years.
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Next up is "Cowboys & Aliens" actress Olivia Wilde at No. 6, followed by Jennifer Lawrence, the Oscar-nominated star of "Winter's Bone," at No. 7. Lawrence cracked the top 100 for the first time this year.
The number of women on this year's list -- five -- is two more than the number on the 2010 list.
For the first time in four years, no "Twilight" actors are on the IMDb list -- despite "Breaking Dawn -- Part 1" being one of the highest-grossing movies of the year.
IMDb Managing Editor Keith Simanton chalked up Portman's topping of the list to her Best Actress Oscar for "Black Swan" and roles in three films in 2011: "No Strings Attached," "Your Highness" and "Thor."
Additionally, Portman was a regular fixture in the tabloids this year, as she had a baby boy in June.
As for Kunis, the "Friends With Benefits" actress's No. 2 spot was perhaps boosted by her attendance at a Marine Corps Ball in November, Simanton said. While stationed in Afghanistan, the solider asked out Kunis through a video on YouTube.