The Justice Department is giving companies 90 days to get accustomed to its recent reversal that would ban most online gambling.

The DOJ’s Office of Legal Counsel (OLC) issued a legal opinion, made public Jan. 14, that the 1961 Wire Act prohibitions apply to non-sports online betting. It’s a reversal from the office’s 2011 opinion, which held that the Wire Act didn’t restrict online gambling for activities other than sporting events. The pre-internet law was designed to crack down on betting through wire communications.

“A 90-day window will give businesses that relied on the 2011 OLC opinion time to bring...