An auto transformer (sometimes called autoformer) is an electrical transformer with only one winding. The winding has at least three electrical connection points called taps.

The voltage source and the load are each connected to two taps. One tap at the end of the winding is a common tap voltage. In an auto transformer a portion of the same winding acts as part of both the primary and secondary winding. It corresponds to a different source or load and connect to both circuits (source and load).

Auto transformers are frequently used in power applications to interconnect systems operating at different voltage classes, for example 138KV to 66KV for transmission. Another application is in industry to adapt machinery built (for example) for 480V supplies to operate on a 600V supply. They are also often used for providing conversions between the two common domestic mains voltage bands in the world (100-130V and 200-250V). The links between the UK 400 kV and 275 kV 'SuperGrid' networks are normally three phase auto transformers with taps at the common neutral end.

On long rural power distribution lines, special auto transformers with automatic tap-changing equipment are inserted as voltage regulators, so that customers at the far end of the line receive the same average voltage as those closer to the source. The variable ratio of the auto transformer compensates for the voltage drop along the line.