Nev Kemp, deputy chief constable of Surrey Police, was found to have "fallen short" of the standards expected of a senior police officer.
He was given a final written warning following a disciplinary hearing held in private last month.
A Surrey Police statement said the warning would remain in place for two years.
It said: “On Monday 29 March an independent panel concluded that Deputy Chief Constable Nev Kemp did breach standards of professional behaviour by not disclosing a workplace relationship that had the potential to cause a conflict of interest, as required by national and local policy.
“The independent panel was made up of a legally qualified chair, a member of Her Majesty’s Inspectorate of Constabulary and an independent member.
“He has been given a final written warning, which will be valid for two years.
“Following the private meeting, a period of time is given to review the panel decision which has now passed.”
Chief Constable Gavin Stephens said: “This is a proportionate outcome. DCC Kemp did fall short of the standards expected of a senior police officer, but through his own admission has accepted his responsibility and has apologised for his actions.
“He remains in his role and is committed to serving the public of Surrey.”