A DSL filter (also DSL splitter or microfilter) is an analog low-pass filter installed between analog devices (such as telephones or analog modems) and a plain old telephone service (POTS) line. The DSL filter prevents interference between such devices and a digital subscriber line (DSL) service connected to the same line. Without DSL filters, signals or echoes from analog devices at the top of their frequency range can reduce performance and create connection problems with DSL service, while those from the DSL service at the bottom of its range can cause line noise and other problems for analog devices.

The concept of a low pass filter for ADSL was first described in 1996 by Vic Charlton when working for the Canadian Operations Development Consortium: Low-Pass Filter On All Phones.

DSL filters are passive devices, requiring no power source to operate. Some high-quality filters may contain active transistors to refine the signal.