What Is a DAT File?
You found a .dat file and Windows won't open it. That is expected: .dat is a generic label, not one format. Here is how to work out what yours actually is and how to open it.
Why there is no single answer
.dat just means "data." It is a generic extension that many different programs use for their own private data, so a .dat from a video game, an email client, and a backup tool have nothing in common. There is no one program that opens all of them — you have to identify what created yours.
The most common case: winmail.dat
By far the most-asked-about .dat is winmail.dat, which arrives as an email attachment. It is Microsoft Outlook's proprietary TNEF wrapper: when an Outlook sender uses Rich Text format, the real attachments get bundled inside winmail.dat, and non-Outlook mail apps can't unpack it. To get your actual files, open winmail.dat with a TNEF reader — for example a free winmail.dat viewer app, or ask the sender to resend using "Plain Text" or "HTML" format instead of Rich Text.
How to identify any other DAT file
Look at where it came from. A .dat inside a game's folder is that game's save or config data — leave it alone. One next to a program's settings is app data. If you have no idea, open it in a plain text editor: many .dat files begin with a readable signature or header (for example media files may show RIFF or a codec name) that reveals the true format, which you can then open with the right program.
Is it safe to delete?
It depends entirely on the source. A stray winmail.dat in your Downloads can be discarded once you have the attachments. A .dat living inside an application's or game's folder may be essential data — deleting it can lose saves or corrupt settings, so don't remove those.
FAQ
How do I open a winmail.dat file?
It is Outlook's TNEF attachment wrapper. Open it with a winmail.dat / TNEF viewer to extract the real files, or ask the sender to resend the email as Plain Text or HTML instead of Rich Text.
Why won't my DAT file open?
Because .dat is not a single format — it is whatever the program that created it decided to store. You must identify the source program first. Opening it in a text editor can reveal a header that hints at the real type.
Is it safe to delete a DAT file?
A stray winmail.dat is safe once you have the attachments. A .dat inside a game or application folder may be a save or settings file — deleting those can lose data, so leave them alone.
Can I open a DAT file in Notepad?
You can try — text-based .dat files will be readable, and binary ones will show a header that may identify the format. It won't hurt the file to look.