The unspecified threat prompted police to scour Sanrio Puroland for suspicious objects, but none were found, public broadcaster NHK reported.
Authorities were trying to identify the sender of the email, NHK added, citing unnamed police sources.
Tokyo-based Sanrio created the ubiquitous pink-bowed character Hello Kitty in 1974, and opened the Sanrio Puroland theme park in Tama City, Tokyo in 1990.
The threat against the popular tourist attraction and its closure sparked groans on social media, with one user annoyed it happened during a holiday weekend.
“It’s unforgivable that someone is doing this during a precious three-day weekend!” the user posted on X, formerly Twitter.