In telecommunications and electrical engineering in general, an unbalanced line is a pair of conductors intended to carry electrical signals, which have unequal impedances along their lengths and to ground and other circuits. Examples of unbalanced lines are coaxial cable or the historic earth return system invented for the telegraph, but rarely used today. Unbalanced lines are to be contrasted with balanced lines, such as twin-lead or twisted pair which use two identical conductors to maintain impedance balance throughout the line. Balanced and unbalanced lines can be interfaced using a device called a balun.

The chief advantage of the unbalanced line format is cost efficiency. Multiple unbalanced lines can be provided in the same cable with one conductor per line plus a single common return conductor, typically the cable shielding. Likewise, multiple microstrip circuits can all use the same ground plane for the return path. This compares well with balanced cabling which requires two conductors for each line, nearly twice as many. Another benefit of unbalanced lines is that they do not require more expensive, balanced driver and receiver circuits to operate correctly.

Unbalanced lines are sometimes confused with single-ended signalling, but these are entirely separate concepts. The former is a cabling scheme while the latter is a signalling scheme. However, single-ended signalling is commonly sent over unbalanced lines. Unbalanced lines are not to be confused with single-wire transmission lines which do not use a return path at all.