TOKYO—Walt Disney Co.’s Japan unit apologized after artists it hired to endorse its “Frozen 2” movie on Twitter failed to label the tweets as promotional.

Seven artists posted Japanese-language cartoons on Twitter reviewing the movie favorably around 7 p.m. Japan time Tuesday, using the same hashtag. None stated they were hired for the work. The next day, after Twitter users remarked on the near-simultaneous posts, the cartoonists tweeted that they should have made clear the works were part of Disney’s marketing.

Walt...