B9 or B-9 may refer to:

• Vitamin B9, folic acid.
• B9, the IATA code for Iranair Tours.
• The Boeing Y1B-9, a bomber of the United States Army Air Corps.
• HMS B9 , a B-class submarine of the Royal Navy.
• B9, an international standard paper size (44×62 mm), defined in ISO 216.