Henry Jacob Friendly (July 3, 1903 – March 11, 1986) was an American lawyer and judge who was a federal circuit judge on the United States Court of Appeals for the Second Circuit from 1959 to 1986. He served as the court's chief judge from 1971 to 1973 and presided over its specialized railroad court from 1974 to 1986.

Born in Elmira, New York, Friendly graduated with highest honors from Harvard College at age 19. He then excelled as a prodigy at Harvard Law School, where he achieved the highest grades in the school's history, was elected president of the Harvard Law Review, and is credited with inventing The Bluebook. After clerking for Justice Louis Brandeis, he co-founded the law firm of Cleary Gottlieb Steen & Hamilton in 1945 and became the general counsel and vice president of Pan Am Airways in 1946. On the recommendation of Judge Learned Hand and Justice Felix Frankfurter, President Dwight Eisenhower appointed Friendly to the Second Circuit in 1959.

Friendly was a prodigious writer who penned more than 1,000 opinions while authoring books and law review articles that are now considered seminal. He was especially influential in the fields of administrative law, securities regulation, and federal jurisdiction. His opinions remain some of the most cited in federal jurisprudence and he is considered one of the most prominent and influential federal judges of the 20th century.

Friendly was born in Elmira, New York, on July 3, 1903, the only child of a middle class German-Jewish family. He was descended from Southern German dairy farmers in Wittelshofen, Bavaria, that had adopted the surname of Freundlich. Josef Myer Freundlich (1803–1880), Friendly's great-grandfather, was a prosperous farmer whose estate burned down in 1831; after being denied help by his neighbors because he was Jewish, Josef grew affluent from livestock dealing. Heinrich Freundlich, Friendly's grandfather, immigrated to the United States in 1852 to avoid conscription and anglicised the family surname to Friendly. Heinrich worked as a businessman in Cuba, New York, beginning as a peddler. He progressed to own a carriage factory before the birth of Friendly's father, Myer Friendly, who migrated to Elmira in his youth.

The Friendlys resided in the primarily Christian, western side of Elmira, opposite of the city's Jewish community. They held various civic positions in town, lived comfortably, and were known as active members of the local German-Jewish population. A monograph in Elmira commemorates Friendly's grandfather, a generous donor to the Jewish community, as "one of the leading men of Elmira in the late nineteenth century." Though not devoutly religious, the family attended a Reform temple alongside other German Jews and held a bar mitzvah for their only son.

Friendly demonstrated precocious abilities in reading and diction at a young age. As early as age seven, "he could read almost any book written for adults." His mother, Leah Hallo, was a bardolater skilled at contract bridge with an excellent memory. She "poured all her attention to her son," headed a local Shakespeare club, and frequently took him to Gilbert and Sullivan performances; he later recalled, "there was absolutely nothing she wouldn't have done for me." Myer, by contrast, was a conservative father who impressed high standards of work and perfection. The marriage began unhappy with Leah choosing to move in with her sister in Chicago, but she later returned. "We didn't have a very close family," Friendly remembered.

As a child, Friendly was known locally for his earnestness. Outside of school, he frequented the outdoors, often walking to Mark Twain's study, and visited a great-aunt who played scores of Richard Wagner. He experienced his first exposure to law while serving as a teenage expert witness in a breach of warranty trial. By means of a friend's father, a lawyer, he developed a respect for the profession. He was a committed reader who enjoyed baseball but was an overweight, unathletic teenage boy. Myer, a sportsman and fisherman, took his son on forays that Henry would ultimately come to reject, which disappointed Myer. Henry also lacked dexterity and struggled with handling objects into adulthood; after puncturing his hand with a pencil, he lost function of his left-hand little finger and contracted a serious case of blood poisoning. Eye problems developed during boyhood, which would advance to retinal detachment in 1936, further complicated his health. A lack of friends, combined with a lack of close relationships, resulted in social and emotional defects that persisted throughout his life.

Although he missed several periods of school away on family vacations, Friendly skipped three grades. He took interests in American history and English literature—especially English writers George Eliot and William Makepeace Thackeray—but avoided science. He became a versatile student at the Elmira Free Academy, where he was considered one of the "most brilliant students ever to attend" and once discovered a mathematical error in its trigonometry textbook. He was chosen to be class valedictorian and editor-in-chief of the academy's newspaper, The Vindex. Upon graduating in 1919, he sat the New York Regents Examinations, attaining the highest scores ever recorded in its 55-year history.