Data file

What is a .nsf file?

A .nsf file is a Lotus Notes Database (version 2). This is a data file — structured information used by a program rather than read directly. 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 .nsf over the years, so two files with the same extension aren't necessarily the same kind of thing.

How to open a .nsf file

Start with the program that produced it; that's almost always the right tool.
Open it in a text editor to see whether it's readable text or binary.
For database formats, DB Browser for SQLite or LibreOffice Base can often import it.

The registry records this extension against Lotus Notes Database, Lotus Notes Database, Lotus Notes Database, so software from that family is the most likely match.

How to convert a .nsf file

If you can view the data, exporting to .csv is usually the most portable option.

MIME type

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

application/vnd.lotus-notes

What's a MIME type?

The technical record

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