Lala Rudra Dayal Mathur (Punjabi: ਲਾਲਾ ਹਰਦਿਆਲ; 14 October 1884 – 4 March 1939) was an Indian nationalist revolutionary and freedom fighter. He was a polymath who turned down a career in the Indian Civil Service. His simple living and intellectual acumen inspired many expatriate Indians living in Canada and the U.S. in their campaign against British rule in India during the First World War.

Har Dayal Mathur was born in a Hindu Mathur Kayastha family on 14 October 1884 in Delhi. He studied at the Cambridge Mission School and received his bachelor's degree in Sanskrit from St. Stephen's College, Delhi and his master's degree also in Sanskrit from Punjab University. In 1905, he received two scholarships of Oxford University for his higher studies in Sanskrit: Boden Scholarship, 1907 and Casberd Exhibitioner, an award from St John's College, where he was studying.

He moved to the United States in 1911, where he became involved in industrial unionism. He had also served as secretary of the San Francisco branch of the Industrial Workers of the World alongside Fritz Wolffheim, (later a National Bolshevik after he had left IWW and joined the Communist Workers' Party of Germany). In a statement outlining the principles of the Fraternity of the Red Flag, he said they proposed "the establishment of Communism, and the abolition of private property in land and capital through an industrial organization and the general strike, ultimate abolition of the coercive organization of government". A little over a year later, this group was given 6 acres (24,000 m²) of land and a house in Oakland, where he founded the Bakunin Institute of California, which he described as "the first monastery of anarchism".

In California, he soon developed contacts with Punjabi Sikh farmers in Stockton. Punjabis, a great majority of whom were Sikhs, had started emigrating to the West Coast around the turn of the century. Having experienced hostility by the Canadians in Vancouver, they had already become disaffected with the British. Hardayal tapped into this sentiment of these energetic Sikhs and other Punjabis. Having developed an Indian nationalist perspective, he encouraged young Indians to gain scientific and sociological education.

In April 1914, he was arrested by the United States government for spreading anarchist literature and fled to Berlin, Germany. In Berlin he became instrumental to the formation of the Berlin Committee (later: Indian Independence Committee) and cooperated with the German Intelligence Bureau for the East.

He died in Philadelphia on 4 March 1939. In the evening of his death, he delivered a lecture as usual where he had said: "I am in peace with all". But a very close friend of Lala Hardayal and the founder member of Bharat Mata Society (established in 1907), Lala Hanumant Sahai, did not accept the death as natural, he suspected it as poisoning.

In 1987, the India Department of Posts issued a commemorative stamp in his honor, within the series of "India's Struggle for Freedom".

Some of his books with available references are listed below: