FBI agents are not unionized. Instead, they are represented by the FBI Agents Association (FBIAA), a non-profit organization that works to advance and safeguard their careers, economic interests, employment conditions, and welfare[1].

Sources [1] Federal Bureau of Investigation Agents Association - FBIAA https://www.fbiaa.org [2] FBIAA - About the FBI Agents Association https://www.fbiaa.org/about/ [3] FBI Employees https://www.fbi.gov/about/fbi-employees [4] AFGE at a Glance https://www.afge.org/about-us/afge-at-a-glance/ [5] A Brief History - FBI https://www.fbi.gov/history/brief-history