Police cordoned off platform 3 of the station on Friday (June 6) at 9.10pm after reports of the substance being sprayed on the side of a tube carriage.
People on the tube were evacuated as a precaution, but there were no reports of any injuries.
According to a British Transport Police spokesperson, it was believed that an alkaline substance was used.
Five police cars were parked outside the station as officers ran to the platform to deal with incident.
No arrests have been made.
While officers investigated, there was no service between Kennington and Moorgate northbound, as Transport for London issued a “security alert”.
A British Transport Police spokesperson said: “Officers were called at around 9.10pm on Friday (6 June) to Kennington Underground station following reports of a substance being sprayed on the side of a tube carriage.
“Specialist officers attended and the train was evacuated as a precaution.
“There were no reports of any injuries.
“Enquiries into the circumstances of the incident are ongoing.”
