The Sussex police and crime commissioner (PCC) is being investigated over alleged false expenses claims during the election cycle earlier this year.
The Independent Police Complaints Commission (IPCC) is looking at complaints made against Katy Bourne.
A member of the public said that Ms Bourne posted on social media during the elections saying she had not claimed any expenses during her first four-year term.
The complainant alleges she filed expense claims of around £385 between November 2012 and October 2013.
IPCC Deputy Chair Sarah Green said: "As Mrs Bourne is a Police and Crime Commissioner, the IPCC will investigate whether she made a false statement during the 2016 election regarding her expenses while serving as the PCC.
"On completion of the investigation we will decide whether any matters should be referred to the Crown Prosecution Service for its consideration."
PCCs are only referred to the IPCC when there is an indication of a possible criminal offence. They then make a decision on whether it is necessary to investigate.
The IPCC will decide whether there has been a criminal breach of s106 of the Representation of the Peoples Act 1983.
Ms Bourne was elected to her first term in 2012 and was re-elected in May this year with 114,570 votes.