Patrick Bamford said he had instructed a French lawyer to pursue criminal and civil proceedings against the player.
He is also taking libel action against a number of news organisations which reported the incident.
The alleged incident occurred at the recent Heineken Cup game between Toulouse and Ulster.
Television footage showed Brennan marching into the crowd and allegedly punching Mr Bamford.
Brennan was warming up on the touchline with a number of other Toulouse substitutes when he alleged he was verbally abused by a number of Ulster fans.
He has since apologised for the incident.
But a statement from Mr Bamford, who lives and works in London, made clear it was not enough.
He said: "Following the violent attack on me a fortnight ago during in which I suffered a number of injuries, I can confirm that I have instructed a lawyer in France to pursue criminal and civil proceedings against Mr Brennan."
He said he was also taking action against a number of media outlets which he accused of publishing false and damaging claims about his role in the incident.
"I am taking legal action against these organisations for the damage they have inflicted on my reputation," he said.
Mr Bamford added: "To attend a rugby match as an innocent spectator and be the victim of this particularly vicious physical assault was extremely shocking in itself, but to then have to be subjected to false claims about my behaviour was the final straw, leaving me with no alternative but to place the matter in the hands of libel lawyer Paul Tweed, to take the appropriate legal action."