The opinion was requested by Gov. Jack Markell, who has signed a bill that would make Delaware the only state east of the Rocky Mountains to offer sports wagering.
In a 22-page ruling dated Wednesday, the court says the state constitution permits lotteries that have an element of skill, as long as chance is the predominant factor in winning or losing.
The justices also say the proposed sports lottery satisfies the constitutional requirement that lotteries be under state control.
The NFL opposes the lottery and has said it may challenge the bill in court.