Insp Neil McGurk added: “This type of behaviour is unacceptable and will not be tolerated.”
Posting on X, formerly Twitter, Raith said the club is in touch with the fan and will support them privately.
It said: “We have contacted the supporter to check on their wellbeing and to see how we can provide support in any way possible.
“The incident has been reported to the police and we will work with Dunfermline Athletic to identify the offenders.”
Dunfermline said it will work with authorities to support the fan and any supporters found to have taken part will be banned from East End Park.
A post on the club’s website adds: “The club condemns all forms of violence in the strongest possible way and we we will support the authorities in identifying any individuals involved.”
Anyone who witnessed the assault has been urged to contact police via the non-emergency line, external.
Ian Murray’s side won 2-1 against Dunfermline, preserving their three-point lead at the Scottish Championship summit.
