The veteran Congress leader was 96. Mr Keishing, a Naga, served as the chief minister of Manipur from 1980 to 1988 and from 1994 to 1997.
Rishang Keishing passed away at around 8:15 pm due to multiple organ failure, doctors attending him at the Regional Institute of Medical Sciences (RIMS) said.
He was hospitalised on July 1 after his health condition worsened.
He is survived by two sons and four daughters. Mr Keishing, who hailed from Bungpa village in the Ukhrul district, was elected to the Lok Sabha in the first Parliamentary elections held in 1952. He had also represented Manipur in Rajya Sabha.
In Delhi, Home Minister Rajnath Singh condoled the demise of Mr Keishing, and described him as a “towering leader” from Manipur who believed in humanity, social harmony and clean politics.
“I am pained by his demise, I offer my heartfelt condolences to the bereaved family of Shri Keishing. My thoughts are with his family in this hour of grief,” he tweeted.
Union minister of state for home Kiren Rijiju also expressed grief. In a Twitter post, Mr Rijiju remarked that despite being 96-year-old, Mr Keishing “remained so fit till date”.
The Manipur government has declared that all government offices and educational institutions will be closed tomorrow as mark of respect to Mr Keishing.
Manipur Governor Najma Heptulla expressed her sadness and condoled the demise of Mr Keishing.
Congress vice president Rahul Gandhi also mourned Rishang Keishing’s demise.
“Ex CM & Parliamentarian Rishang Keishingji was a true Congressman & one of the tallest leaders of North East.Very sad to know of his passing (sic),” said a post on Mr Gandhi’s official Twitter handle.