The deal gives USJ an enterprise value of $6.2 billion, Comcast said.
USJ had planned a Tokyo listing this year, but the timing of that has become uncertain due to concerns about being crowded out by the $11.5 billion initial public offerings of JapanPost group companies in November, two sources familiar with the matter have told Reuters.
Comcast Chief Executive Brian Roberts told reporters there were no plans for an IPO in the near term, but added a new board would revisit the idea.
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USJ opened its theme park in 2001 and listed on the Tokyo bourse's Mothers Market for start-ups in 2007. It had hoped to position itself as a rival to Tokyo Disneyland but failed to gain traction. It was delisted in 2009 after struggling with falling visitor numbers and was sold to a consortium led by Goldman Sachs.
But USJ is now enjoying solid business on the back of its "Wizarding World of Harry Potter," which opened July last year as well as other attractions, and also plans to build a new theme park in Okinawa.
It had its best year in the last financial year ended in March with 12.7 million visitors, though that trails the 31.4 million visitors to Tokyo's Disney Resort, which includes Disneyland and DisneySea.
Comcast said it expects to close the deal in the first half of November.