The 31-year-old, who gave birth to son Aleph in June 2011, was watching the game in Austin, Texas during a break from shooting the as-yet-untitled new Terrence Malick film.
With co-star Michael Fassbender by her side, a newly blonde Natalie was spotted by the cameras during the ABC broadcast.
While it could very well be a push-up bra, or a consequence of breast-feeding her bub, Twitter fans immediately speculated about the star's potentially enhanced assets.
"Natalie Portman @ the Texas-Baylor game ... boob job? Looks like it!" one user wrote.
"Wait a second, did Natalie Portman get a boob job? Is she pregnant? On the sidelines of Texas game and #wow," another tweeted.
Comments on the YouTube clip ran along the same lines, albeit many of them censored, with dozens of "this has been flagged as spam" messages.
One of the posts that got through was the straight-shooting zinger, "Fake t--- are awesome!"
So, did she or didn't she? Her chest does indeed look (how do we put this delicately?) unusually buoyant? Compare the before and after shots and cast your vote below.
Related pics: When celebrity boob jobs go wrong …
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