Yes, Niagara Falls is open on Christmas Day. The falls are actually open year-round, 24 hours a day. So no matter when you visit, you'll be able to see the falls in all their glory.
One of the best things about visiting Niagara Falls on Christmas Day is that there are usually fewer crowds. So if you're looking to avoid the crowds, this is a great time to visit. That said, it's still a popular spot on Christmas Day, so you may want to get there early to beat the crowds.
No matter when you visit, you're sure to be impressed by the majesty of Niagara Falls. So if you find yourself in the area on Christmas Day, be sure to check it out.