Inspector General Michael Horowitz said he had broken "dramatically from FBI and department norms" in handling an inquiry into Hillary Clinton's emails.
Mrs Clinton has blamed Mr Comey for her election loss to Donald Trump in 2016.
The report also reveals Mr Comey used a private email account to conduct official FBI business.
FBI director Christopher Wray said he accepted the report's findings but he added that nothing in the report pointed towards political bias or impugned the FBI as an institution.
In his report, the inspector general criticised Mr Comey's decision to reveal publicly a week before the election that he had reopened the inquiry into Mrs Clinton's emails, rejecting Mr Comey's argument that he had acted in the interests of transparency.
He found that while Mr Comey's actions were not the result of political bias, "by departing so clearly and dramatically from FBI and department norms, the decisions negatively impacted the perception of the FBI and the department as fair administrators of justice".
The 500-page report also found "a troubling lack of any direct, substantive communication" between Mr Comey and Former Attorney General Loretta Lynch.
But the report also touched on text messages between two FBI officials who later worked on Special Counsel Robert Mueller's investigation into alleged Russian meddling in the US election.
Peter Strzok, who was Mr Mueller's lead agent in the Russian inquiry, was having an affair with Lisa Page, an FBI lawyer who also temporarily worked on the Mueller investigation.
When Ms Page asked if Mr Trump would become president, Mr Strzok responded: "No. No he won't. We'll stop it."
The report called this "not only indicative of a biased state of mind but, even more seriously, implies a willingness to take official action to impact the presidential candidate's electoral prospects".
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