If you are using official Netgate hardware, the Serial Number will be clearly listed here.

You can also try running kenv to see various kernel environment variables that often include hardware identifiers.

If you are connected via SSH or a console cable, you can pull the serial number directly from the system environment using these commands:

The pfSense serial number is a unique identifier primarily used to identify hardware appliances for support, warranty, and software registration. While it is most critical for official Netgate hardware, even custom builds may display a serial number or a host UUID depending on the system's BIOS. Where to Find Your pfSense Serial Number

The easiest way to find the serial number is through the pfSense WebGUI dashboard .

Run dmidecode -t system | grep Serial to pull the serial number directly from the hardware BIOS.