Because hemangiosarcoma tumors most often in develop in internal organs, frequently, there are few or no  obvious symptoms before the onset of  severe clinical signs of disease.  Signs of this disease are usually the result of the tumor rupturing, which causes bleeding. This may occur without any warning, and the symptoms will depend upon where the tumor is located.  The most common symptom will be a lump under the skin, visible bleeding, sometimes in the form of nosebleeds, tiring easily, episodes of unexplained weakness, pale color in the gums, difficulty breathing, abdominal swelling, seizures, abnormal heart rhythms, collapse, and depression.

When the tumor is located in the spleen or liver, the clinical signs are usually due to rupture of the tumor and subsequent bleeding into the abdomen. This causes anemia, weakness and if the bleeding is severe, collapse. The gums may appear to be pale or white.  In relative few dogs, the diagnosis is made before the tumor ruptures.
When the  tumor is located in the heart, it can cause symptoms, such as weakness, collapse, difficulty breathing, exercise intolerance and fluid build-up in the abdomen. This is usually due to the development of fluid around the heart, called pericardial effusion. The pericardium is a thin sack that surrounds the heart and  with hemangiosarcoma, the pericardium fills up with blood due to rupture of the tumor.

When the tumor occurs in the skin, a mass or lump can usually be felt in or under the skin. The mass may become ulcerated and bleed. When the tumor occurs in a bone, it can cause pain and discomfort. In some locations, such as a rib, the tumor can be felt as a firm swelling in the bone.