Jacob August Riis (/riːs/ REESS; May 3, 1849 – May 26, 1914) was a Danish-American social reformer, "muck-raking" journalist, and social documentary photographer. He contributed significantly to the cause of urban reform in the United States of America at the turn of the twentieth century. He is known for using his photographic and journalistic talents to help the impoverished in New York City; those impoverished New Yorkers were the subject of most of his prolific writings and photography.

Riis endorsed the implementation of "model tenements" in New York with the help of humanitarian Lawrence Veiller. He was an early proponent of the newly practicable casual photography and one of the first to adopt photographic flash. While living in New York, Riis experienced poverty and became a police reporter writing about the quality of life in the slums. He attempted to alleviate the poor living conditions of poor people by exposing these conditions to the middle and upper classes.

Born in May 3, 1849 in Ribe, Denmark, Jacob Riis was the third of 15 children (one of whom, an orphaned niece, was fostered) of Niels Edward Riis, a schoolteacher and writer for the local Ribe newspaper, and Carolina Riis (née Bendsine Lundholm), a homemaker. Of the 15, only Jacob, one sister, and the foster sister survived into the twentieth century. Riis was influenced by his father, whose school Riis delighted in disrupting. His father persuaded him to read and improve his English via Charles Dickens's magazine All the Year Round and the novels of James Fenimore Cooper.

Jacob reportedly had a happy childhood, but experienced tragedy at the age of eleven when his brother Theodore, a year younger, drowned. He never forgot his mother's grief.

At age eleven or twelve, he donated all the money he had to a poor Ribe family, living in a squalid house if they cleaned it. The tenants took the money and obliged. When he told his mother, she went to help.

Though his father hoped that Jacob would have a literary career, Jacob wanted to be a carpenter. When he was 16, he became fond of Elisabeth Gjørtz, the 12-year-old adopted daughter of the owner of the company for which he worked as an apprentice carpenter. The father disapproved of the boy's blundering attentions, and Riis was forced to travel to Copenhagen to complete his carpentry apprenticeship. In 1868, Riis returned to Ribe at age 19. Discouraged by poor job availability in the region and Gjørtz's disfavor of his marriage proposal, Riis decided to emigrate to the United States.

Riis immigrated to America in 1870, when he was 21 years old, seeking employment as a carpenter. He first traveled in a small boat from Copenhagen to Glasgow, where on May 18 he boarded the steamer Iowa, traveling in steerage. He carried $40 donated by friends (he had paid $50 for the passage himself); a gold locket with a strand of Elisabeth's hair, presented by her mother; and letters of introduction to the Danish Consul, Mr. Goodall (later president of the American Bank Note Company), a friend of the family since his rescue from a shipwreck at Ribe. Riis disembarked in New York on June 5, on that day spending half of the $40 his friends had given him on a revolver for defense against human or animal predators.

When Riis arrived in New York City, he was one of a large number of migrants and immigrants, seeking prosperity in a more industrialized environment, who came to urban areas in the years after the American Civil War. Twenty-four million people relocated to urban areas, causing their population to increase eightfold. The demographics of American urban areas became significantly more heterogeneous as many immigrants arrived, creating ethnic enclaves often more populous than many of the cities of their homelands. "In the 1880s 334,000 people were crammed into a single square mile of the Lower East Side, making it the most densely populated place on earth. They were packed into filthy, disease-ridden tenements, 10 or 15 to a room, and the well-off knew nothing about them and cared less."