Ishwar Chandra Vidyasagar CIE (Bengali: ঈশ্বর চন্দ্র বিদ্যাসাগর Ishshor Chôndro Biddashagor 26 September 1820 – 29 July 1891), born Ishwar Chandra Bandopadhyaya (Bengali: ঈশ্বর চন্দ্র বন্দ্যোপাধ্যায়, Ishshor Chôndro Bôndopaddhae), was an Indian Bengali polymath and a key figure of the Bengal Renaissance.

Vidyasagar was a philosopher, academic, educator, writer, translator, printer, publisher, entrepreneur, reformer, and philanthropist. His efforts to simplify and modernize Bangla prose were significant. He also rationalized and simplified the Bengali alphabet and type, which had remained unchanged since Charles Wilkins and Panchanan Karmakar had cut the first wooden Bangla type fonts in 1780. He received the title "Vidyasagar" ("Ocean of learning" or "Ocean of knowledge") from the Calcutta Sanskrit College (where he graduated), due to his excellent performance in Sanskrit studies and philosophy. In Sanskrit, Vidya means knowledge or learning and Sagar means ocean or sea. This title was mainly given for his vast knowledge in all subjects which was compared to the vastness of the ocean.

Iswar Chandra Vidyasagar was born at Birsingha village, in the Ghatal subdivision of Midnapore District, on 26 September 1820 to a poor religious family. Actually, Birsingha is now a village in the Ghatal subdiviison of Pashchim Medinipur district, but at the time when Vidyasagar was born, this village was part of then Hooghly district. His parents were Thakurdas Bandyopadhyay and Bhagavati Devi. The childhood days of Vidyasagar were spent in abject poverty. After the completion of elementary education at the village school, his father took him to Calcutta. Ishwar Chandra was a brilliant student. It is believed that Ishwar Chandra Vidyasagar learned English numbers by following the milestone labels on his way to Calcutta at the age of eight years. His quest for knowledge was so intense that he used to study under street light as it was not possible for him to afford a gas lamp at home. He passed all the examinations with excellence and in quick succession. He was rewarded with a number of scholarships for his academic performance. To support himself and the family Ishwar Chandra also took a part-time job teaching at Jorashanko.

In the year 1839, Ishwar Chandra Vidyasagar successfully passed his Law examination. In 1841, at the age of twenty one years, Ishwar Chandra joined the Fort William College as head of the Sanskrit department.

After five years, in 1846, Vidyasagar left Fort William College and joined the Sanskrit College as 'Assistant Secretary'. In the first year of service, Ishwar Chandra recommended a number of changes to the existing education system. This report resulted into a serious altercation between Ishwar Chandra and College Secretary Rasomoy Dutta. In 1849, he again joined Sanskrit College, as a professor of literature. In 1851, Iswar Chandra became the principal of Sanskrit College. In 1855, he was made special inspector of schools with additional charges. But following the matter of Rasomoy Dutta, Vidyasagar resigned from Sanskrit College and rejoined Fort William College but as a head clerk.

Vidyasagar in Calcutta and many other reformers in Bombay set up schools for girls. When the first schools were opened in the mid nineteenth century, many people were afraid of them. They feared that schools would take away girls from home and prevent them from doing their domestic duties. Moreover, girls would have to travel through public places in order to reach school. They thought that girls should stay away from public places. Therefore, most educated women were taught at home by their liberal fathers or husbands.

In 1841, Vidyasagar took the job of a Sanskrit pandit (professor) at Fort William College in Kolkata (Calcutta). In 1846, he joined the Sanskrit College as Assistant Secretary. A year later, he and a friend of his, Madan Mohan Tarkalankar, set up the Sanskrit Press and Depository, a print shop and a bookstore.

While Vidyasagar was working at the Sanskrit College, some serious differences arose between him and Rasamoy Dutta who was then the Secretary of the College, and so he resigned in 1849. One of the issues was that while Rasamoy Dutta wanted the College to remain a Brahmin preserve, Vidyasagar wanted it to be opened to students from all castes. Later, Vidyasagar rejoined the College, and introduced many far-reaching changes to the College's syllabus. In the face of opposition from the Hindu establishment, Vidyasagar vigorously promoted the idea that regardless of their caste, both men and women should receive the best education.

Ishwar Chandra Vidyasagar would start crying in distress whenever he saw poor and weak people lying on the footpath and street. Though he was very outspoken and blunt in his mannerisms, yet Vidyasagar had a heart of Gold. He was also known for his charity and philanthropy as "Daya-r sagar" - ocean of kindness, for his immense generosity. He always reflected and responded to distress calls of the poor, sufferings of the sick and injustice to humanity. While being a student at Sanskrit College, he would spend part of his scholarship proceeds and cook paayesh (rice pudding) to feed the poor and buy medicines for the sick. Later on, when he started earning, he paid fixed sums of monthly allowances to each member of his joint family, to family servants, to needy neighbours, to villagers who needed help and to his village surgery and school. This he continued without break even when he was unemployed and had to borrow substantially from time to time.

Vidyasagar did not believe that money was enough to ease the sufferings of humanity. He opened the doors of the Sanskrit College to lower caste students (previously it was exclusive to the Brahmins), nursed sick cholera patients, went to crematoriums to bury unclaimed dead bodies, dined with the untouchables and walked miles as a messenger man to take urgent messages to people who would benefit from them.

When the eminent Indian Poet of the 19th century, Michael Madhusudan Dutta, fell hopelessly into debts due to his reckless lifestyle during his stay in Versailles, France, he appealed for help to Vidyasagar, who laboured to ensure that sums owed to Michael from his property at home were remitted to him and sent him a large sum of money to France.

Vidyasagar championed the uplift of the status of women in India, particularly in his native Bengal. Unlike some other reformers who sought to set up alternative societies or systems, he sought, however, to transform orthodox Hindu society "from within".

With valuable moral support from people like Akshay Kumar Dutta, Vidyasagar introduced the practice of widow remarriages to mainstream Hindu society. In earlier times, remarriages of widows would occur sporadically only among progressive members of the Brahmo Samāj. The prevailing deplorable custom of Kulin Brahmin polygamy allowed elderly men — sometimes on their deathbeds — to marry teenage or even prepubescent girls, supposedly to spare their parents the shame of having an unmarried girl attain puberty in their house. After such marriages, these girls would usually be left behind in their parental homes, where they might be cruelly subjected to orthodox rituals, especially if they were subsequently widowed. These included a semi starvation diet, rigid and dangerous daily rituals of purity and cleanliness, hard domestic labour, and close restriction on their freedom to leave the house or be seen by strangers. Unable to tolerate the ill treatment, many of these girls would run away and turn to prostitution to support themselves. Ironically, the economic prosperity and lavish lifestyles of the city made it possible for many of them to have quite successful careers once they had stepped out of the sanction of society and into the demi-monde. In 1853 it was estimated that Calcutta had a population of 12,718 prostitutes and public women.

Vidyasagar took the initiative in proposing and pushing through the Widow Remarriage Act XV of 1856 in India. He also demonstrated that the system of polygamy without restriction was not sanctioned by the ancient Hindu Shastras.

Vidyasagar reconstructed the Bengali alphabet and reformed Bengali typography into an alphabet (actually abugida) of twelve vowels and forty consonants. Vidyasagar contributed significantly to Bengali and Sanskrit literature.

Rectitude and courage were the hallmarks of Vidyasagar's character, and he was certainly ahead of his time. In recognition of his scholarship and cultural work the government designated Vidyasagar a Companion of the Indian Empire (CIE) in 1877. In the final years of life, he chose to spend his days among the "Santhals", an old tribe in India.

Shortly after Vidyasagar's death, Rabindranāth Tāgore reverently wrote about him: "One wonders how God, in the process of producing forty million Bengalis, produced a man!"

Vidyasagar Setu (commonly known as the Second Hooghly Bridge), is a bridge over the Hooghly River in West Bengal, India. It links the city of Howrah to its twin city of Kolkata. The bridge is named after Ishwar Chandra Vidyasagar.

A fair named Vidyasagar Mela (Bengali: বিদ্যাসাগর মেলা Biddashagor Mêla), which is dedicated to spreading education and increasing social awareness, has been held annually in West Bengal since 1994. Since 2001, it has been held simultaneously in Kolkata and Birsingha.

There is a reputed college named after him and it is located in college street, Kolkata, and a university in Paschim Midnapore.