Mustard seeds — Food safety facts
Do not feed · Dogs · full breakdown on the linked food page
Seeds and powder are toxic — they can cause gastroenteritis; high doses may damage the heart.
Short answer
No — mustard seeds are not safe for dogs. The seeds and powder are toxic and can cause vomiting, diarrhea, and other signs of gastroenteritis; in larger amounts they may even damage the heart. If your dog has eaten mustard seeds, call your veterinarian or the ASPCA Poison Control line at 888-426-4435 right away and watch closely for any signs of illness.