Why is my dog throwing up yellow foam?

Yellow foam is usually bile. It often happens when a dog's stomach is empty for a long time, typically early in the morning.

If it's once and your dog is otherwise normal, try a small meal late in the evening or splitting food into more meals.

See a vet if: it keeps happening, your dog vomits several times in a day, won't eat, has diarrhea, seems tired or has a sore belly. Repeated retching with nothing coming up and a swollen belly is an emergency (possible bloat).