Fox Corp. said commercial time during Super Bowl LIV is sold out, as a strong economy and marketers’ appetite for live sports fueled better-than-expected demand.

Advertisers are paying up to $5.6 million for 30 seconds of airtime during the football game, although companies that buy multiple spots do get a discount. Prices for ad time during the last Super Bowl reached as high as $5.3 million.

For...