MHTML, an initialism of "MIME encapsulation of aggregate HTML documents", is a Web archive file format used to combine, in a single computer file, the HTML code and its companion resources (such as images) that are represented by external hyperlinks in the web page's HTML code. The content of an MHTML file is encoded using the same techniques that were first developed for HTML email messages, using the MIME content type multipart/related. MHTML files use an .mhtml or .mht filename extension.

The first part of the file is an e-mail header. The second part is normally HTML code. Subsequent parts are additional resources identified by their original uniform resource locators (URLs) and encoded in base64 binary-to-text encoding. MHTML was proposed as an open standard, then circulated in a revised edition in 1999 as RFC 2557.

The .mhtml (Web archive) and .eml (email) filename extensions are interchangeable: either filename extension can be changed from one to the other. An .eml message can be sent by e-mail, and it can be displayed by an email client. An email message can be saved using a .mhtml or .mht filename extension and then opened for display in a web browser or for editing other programs, including word processors and text editors.