NEW DELHI — Mahasweta Devi, a prominent Bengali writer and social activist who immersed herself in the lives of India’s poor and marginalized as she chronicled the injustices against them in fiction, died Thursday in Kolkata. She was 90.

The cause was a heart attack and multiple organ failure, her grandson, Tathagata Bhattacharya, said.

Ms. Devi had cast off the trappings of the middle class she was born into and chose to live simply as a writer, often roaming the country with her subjects as she did research. In more than 100 novels and short stories, she wrote of India’s tribal communities and Maoist rebels, prostitutes and nomads, beggars and laborers.

In 1997 she earned the Ramon Magsaysay Award, which has been called the “Asian Nobel Prize,’’ for her writing and activism on behalf of tribal communities.

“Her writing addressed one single word: injustice,’’ G.N. Devy, a writer and activist who worked closely with Ms. Devi, said. “Wherever she saw what she thought was injustice, she plunged into the struggle and never looked back.’’

Mahasweta Devi was born in 1926 in what is now Bangladesh to Manish Ghatak, a novelist, and Dharitri Devi, a social activist. She was steeped in service from an early age, observing her mother and aunt educating illiterate girls in Dhaka, where she spent part of her childhood.

She married playwright Bijon Bhattacharya in 1947, and the couple lived in Calcutta, where they had a son.

She described her early married life as one of poverty, and she worked odd jobs to supplement her husband’s income.

She wrote her first novel, “The Queen of Jhansi,’’ at 30. It is a fictionalized account of a real-life queen turned warrior who dressed as a man and fought a doomed war against the British in the Indian Rebellion of 1857, which toppled the Mughal empire.