Yes, a Royal Flush is a guaranteed win in standard poker games like Texas Hold’em. Since it is the highest possible hand. The only exception is if multiple players have it, in which case the pot is split between them.
Disclaimer: It is important to remember that In lowball poker games like 2-7 Triple Draw and Badugi, a Royal Flush is not an automatic winning hand. In these games, the lowest possible hand wins, so it would actually be one of the worst hands you could make.