mb, Mb, and MB may stand for:

• Bachelor of Medicine (academic degree)
• Honda Mb is a Honda 50cc motorcycle from the early 80's.
• Manitoba
• a manned base
• Marching Band (mb), A group of people that play instruments while marching in unison to form some kind of picture or shape.
• Medical Board
• megabit (Mb)¹, or 1,048,576 (2²⁰) bits, or 1,000,000 bits
• megabyte (MB)¹, or 1,048,576 (2²⁰) bytes, or 1,000,000 bytes
• Mercedes Benz
• Meridian and Bigbee Railroad
• millibar (mb)¹, a unit of pressure equal to one hectopascal
• Milton Bradley

¹ Although this is correct usage, capitalisation is often incorrect or completely ignored in many documents, so that the context is more of a clue to what each means.