Yes — but remove the seeds and core first.
Apple flesh is a healthy, crunchy treat full of vitamins. The seeds contain amygdalin, which converts to cyanide — a few seeds won't harm a large dog, but make it a habit to core the apple first, especially for small dogs.
Proceed carefully and follow the guidance above. When in doubt, skip it — there are plenty of unambiguously safe treats to choose from.
Get the printable list of the 30 most dangerous foods for dogs.