An ultra-thin speaker includes a metal upper casing, and a metal lower casing corresponding to the metal upper casing and capable of covering a vibrating membrane, a support frame, a magnet and a yoke between two corresponding internal surfaces of the metal upper and lower casings and forming a contact surface with the yoke, such that after the yoke conducts heat of a heat source to the metal lower casing, the heat is conducted to the metal upper casing through a connection and a contact of the metal upper and lower casings to increase a heat dissipating area, so as to overcome the heat dissipation problem and minimize the overall thickness of the speaker.