Ms Poultney was recently appointed as Deputy Chief Constable following the departure of Mark Roberts, who was appointed Chief Constable of the Cheshire Constabulary.
Her new role came into effect on Monday.
Dr Alan Billings, South Yorkshire Police and Crime Commissioner, said: “As police and crime commissioner it is my responsibility to recruit and appoint a Chief Constable. The recruitment process for a permanent successor to Stephen Watson is currently ongoing. I am delighted to appoint Lauren as temporary Chief Constable to take charge of the force until a permanent appointment has been made.
“Lauren is a very experienced police officer who has been part of the leadership team that has guided South Yorkshire Police from being a force in need of improvement to one that is outstanding in many areas. I know that she will provide excellent leadership to the Force over the coming weeks until a permanent Chief Constable is appointed.”
Detective Chief Constable Poultney joined the force as Assistant Chief Constable in 2017 from Humberside Police.
She has been a police officer for 23 years and has undertaken both uniformed and detective roles in a range of areas, including neighbourhood policing, major crime and protecting vulnerable people.
Assistant Chief Constable Tim Forber will be acting as temporary Deputy Chief Constable until the Chief Constable appointment has been made.