B8 or B-8 may refer to:

• Vitamin B8, sometimes used for either biotin or folic acid.
• B8, a Mazda piston engine.
• B8, the IATA code for Eritrean Airways .
• The Fokker XB-8, a bomber prototype built for the United States Army Air Corps.
• HMS B8 , a B-class submarine of the Royal Navy.
• B8, an international standard paper size (62×88 mm), defined in ISO 216.