Yes — seedless flesh only.
Watermelon flesh is hydrating and safe, but remove all seeds and the rind. Seeds can cause blockages and the tough rind upsets stomachs. Seedless varieties make this easy.
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.