Place a large pot over medium heat and add the ground beef. Cook until browned, breaking it up with a spoon as it cooks. Drain excess fat if needed.
Add the onion, carrots, celery, and garlic to the pot. Stir and cook for 5 minutes until the vegetables start to soften.
Pour in the diced tomatoes with their juice, beef broth, pearl barley, and dried thyme. Season with salt and pepper.
Bring the soup to a boil, then reduce heat to low. Cover and simmer for 45 minutes, stirring occasionally, until the barley is tender.
Taste and adjust salt and pepper before serving.