What is a .sit file?

A .sit file is a Stuffit Archive File (version 5). This is an archive — a container holding other files, usually compressed. Confusingly, this extension has been used by 3 different formats over the years — they're all listed below.

This extension has been used by more than one format

3 different formats have claimed .sit over the years, so two files with the same extension aren't necessarily the same kind of thing.

How to open a .sit file

7-Zip (Windows) or Keka / The Unarchiver (Mac) extract most archive formats, including old ones.
On Linux, try `7z x`, `unar` or Ark.

The registry records this extension against Stuffit Archive File, Stuffit Archive File, Stuffit Archive File, so software from that family is the most likely match.

How to convert a .sit file

Extract the contents, then re-compress as .zip if you need to share them widely.

MIME type

When a .sit file travels over the web, servers label it with this MIME type:

application/x-stuffit

What's a MIME type?

The technical record

For archivists, developers and the merely curious: these are the PRONOM registry entries for .sit.