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.