Can a headache be the only symptom/sign of a brain tumor?

Yes: That is possible. It would depend on the location of the tumor and more factors.

Yes, but..: Headaches are common, and brain tumors aren't. So the most likely cause of a headache is not a brain tumor! warning signs for headaches caused by brain tumors are early morning headaches (upon awakening) or headaches that worsen with coughing or straining. Other causes of headaches: sinus congestion, caffeine withdrawal, muscle tension, eye strain.