The Handheld Device Markup Language (HDML) is used for creating Web pages for portable electronic devices such as smartphones and PDAs. HDML files are similar to .HTML files, but are designed for handheld devices with small displays.

HDML files are not supported by standard Web browsers. However, the HDML code can be viewed and edited with a text editor. HDML Web pages can only be viewed with compatible devices using the UP.Browser program.

The Handheld Device Markup Language (HDML) was developed by Unwired Planet (now Openwave) in 1996. It has since been replaced by the .WML standard.