The utility model provides an adapter and a lighting device. The adapter (200) of an execution mode comprises a tabular body (201) and a positioning protrusion (210), wherein an existing installing hole is blocked by the tabular body (201) and an opening is formed in a ceiling (C) under the condition of the existing installing hole when existing lighting tool is disassembled, and the positioning protrusion (210) is arranged on the surface of the tabular body at one side far away from the ceiling in a protruding mode. A part of a frame (1) of the newly installed lighting tool (100) protrudes to a part of the rear face of a base plate (21) which is provided with a light-emitting component (22) so as to form a contacting protrusion (12). As a result, an embedded concave portion (120) is formed at the rear side of the contacting protrusion. Moreover, the positioning protrusion of the adapter is embedded in the embedded concave portion which is formed at the rear face side of the newly installed lighting tool so that the newly installed lighting tool is positioned at and assembled with the adapter and then the newly installed lighting tool and the adapter are together installed on the ceiling.