A spokesperson for West Sussex Fire & Rescue Service said firefighters were called to the derelict site in Worthing town centre at around 6.15pm on Friday evening (May 15).
One fire engine from Worthing Fire Station was sent.
Once it arrived, it became clear the small fire had begun in a tent at the site.
The fire was extinguished, and all people were accounted for.
During the last several years, the site has been used periodically by people sleeping rough.
In 2017, the owners of the site evicted the homeless community there and hired a security team to prevent them coming back.