A 37-year-old police constable from the Diplomatic Protection Group, which is tasked with protecting diplomatic and government officials, was arrested at her place of work on suspicion of misconduct in a public office, the Metropolitan Police said.
A 46-year-old woman was also arrested at her place of work on suspicion of assisting the commission of an offence of misconduct in a public office. Both women remain in custody.
The police officer arrested is one of the four officers previously issued with a Regulation 15 notice, and has been on restricted duties.
The plebgate row ignited when Mr Mitchell was accused of a heated rant against officers as he left Downing Street on 19 September last year.
Pressure intensified when details of a police log of the incident was published, which claimed he called officers “plebs” and swore at them repeatedly.
He insisted he did not use the words attributed to him and that he was the victim of a deliberate attempt to ruin his career.