Police say a number of searches took place in the Creggan area on Thursday linked to dissident republican activity.
The firearm and suspected ammunition were seized as part of the planned raids.
The 29-year-old author was shot dead by a new ‘New IRA’ gunman while observing riot at Fanad Drive on Thursday, April 18 this year.
Ms McKee, who had only recently moved to Derry to be with her partner Sara Canning, was standing by an armoured police land rover when a gunman opened fire from the shadows on police, before a bullet struck her to the head.
A number of items are believed to have been recovered by the PSNI during searches yesterday.
A 52-year-old man was arrested by detectives from the PSNI’s Terrorist Investigation Unit under the Terrorism Act as part of a probe into violent dissident republican terrorist activity in the city.