The agency, which has offices in Washington DC, New York, San Francisco and London, works for some of the biggest progressive organisations including Amnesty International, Wikileaks and The Guardian.
The news of its closure was first reported in The Huffington Post, which revealed that a number of female staff had come forward with complaints against FitzGibbon.
FitzGibbon sent a statement to The Huffington Post, which read: "It is with tremendous regret that we had to close FitzGibbon Media. Our team was at the helm of many of the most important policy debates of our time. We were the undisputed communications leader for the progressive movement. Although the company is closing, our talents and our mission remain unchanged.
"The allegations against me are a distraction to the mission at hand. In order to do what is best for the company, I took a leave of absence. However, it is abundantly clear that an irreconcilable difference has arisen between the FitzGibbon team and me. We had no choice but to make the difficult decision to close FitzGibbon Media.
"I apologise to my team and our clients for the impact this closing will have on them. I will work to once again regain the trust that was lost."
FitzGibbon opened in the US in 2008 and launched its London office in January this year, headed up by Suzie Gilbert.
When PRWeek contacted FitzGibbon Media, an automated email confirmed it has closed.