Nick Adderley is the most senior police officer in the Northamptonshire force, which is investigating the death of 19-year-old Harry Dunn.
He died when his motorbike hit a car near RAF Croughton in Northamptonshire.
The diplomat's wife, who is a suspect in the case, left the UK despite telling police she did not plan to.
Mr Adderley was asked on Twitter whether the woman was lawfully entitled to claim diplomatic immunity.
He replied: "The short answer is yes," adding that both he and Northamptonshire Police and Crime Commissioner Stephen Mold had written "in the strongest terms" to the US Embassy, urging that the diplomatic immunity waiver be applied "in order to allow the justice process to take place".
Mr Dunn's mother, Charlotte Charles, has told the BBC the family has been left "utterly devastated" by the death of the teenager, from Charlton, Banbury, on 27 August.
On Saturday the US State Department said that diplomatic immunity was "rarely waived" but Foreign Secretary Dominic Raab urged the US Embassy to reconsider.
Mrs Charles told the BBC: "We're really hoping to try to get her back; from me, as a mum, to her, as a mum, you just hope that he [Mr Raab] can try to get through to her.
"We don't wish her any ill harm, but we don't understand how she can just get on a plane and leave our family just utterly devastated.
"If we don't get any luck over here, then we will go over there."
Under the 1961 Vienna Convention, diplomats and their family members are immune from prosecution in their host country, so long as they are not nationals of that country. However, their immunity can be waived by the state that has sent them.
Police said that the suspect "engaged fully" following the crash near RAF Croughton, a US Air Force communications station, and said "she had no plans to leave the country in the near future".