Ronan Daly, representing the coroner’s office, said it was understood the witness had information relating to whether or not Captain Robert Nairac was there on the night in January 1976 when 10 Protestant workmen were murdered.
The lawyer informed the court of the development at a review inquest hearing in Belfast.
“We have been notified in correspondence that the Ministry of Defence has located another witness who attended the scene at Kingsmills,” he said.
Capt Nairac was abducted from a bar in South Armagh the following year and murdered by the IRA.
The inquest has previously heard claims the soldier was involved in the Kingsmills killings, however this was subsequently disputed by a Ministry of Defence lawyer who insisted the soldier was not in the area at the time.
The incident unfolded when workmen in a minibus were stopped by gunmen near the village of Kingsmills in rural South Armagh.
Last year an inquest into the killings was opened but was halted after just one month in June 2016 when the PSNI arrested a man in connection with the case.
However, the decision was made not to prosecute the man due to insufficient evidence.
The inquest was subsequently resumed this year. Proceedings have now been adjourned until November 6.