Arsenic trioxide, sold under the brand name Trisenox among others, is an inorganic compound with the formula As
₂O
₃. As an industrial chemical, its major uses include the manufacture of wood preservatives, pesticides, and glass. It is also used as a medication to treat a type of cancer known as acute promyelocytic leukemia. For this use it is given by injection into a vein.

Common side effects include vomiting, diarrhea, swelling, shortness of breath, and headaches. Severe side effects may include APL differentiation syndrome and heart problems. Use during pregnancy or breastfeeding may harm the baby. Its mechanism in treating cancer is not entirely clear.

Arsenic trioxide was approved for medical use in the United States in 2000. It is on the World Health Organization's List of Essential Medicines. Approximately 50,000 tonnes are produced a year. Due to its toxicity, a number of countries have regulations around its manufacture and sale.