No, a crab is not a mammal. Crabs are actually classified as crustaceans, which are a group of animals that also includes lobsters, shrimp, and other similar creatures.
Mammals are warm-blooded vertebrates that typically have fur or hair and produce milk to feed their young. They belong to the class Mammalia. In contrast, crabs are cold-blooded invertebrates that have an exoskeleton, which is a hard outer shell that protects their body.
Crabs breathe through gills, while mammals have lungs. The differences in their biological makeup and classifications clearly show that crabs are not mammals but rather belong to a completely different category of animals.