Justin Daniel Kaplan (September 5, 1925– March 2, 2014) was an American writer and editor. The general editor of Bartlett's Familiar Quotations (16th and 17th eds.), he was best known as a biographer, particularly of Samuel Clemens, Lincoln Steffens, and Walt Whitman.

Kaplan was born to an Orthodox Jewish family in Manhattan, the son of Tobias D. Kaplan, a successful shirt manufacturer in New York City, and Anna (Rudman) Kaplan, a homemaker. Both of his parents died by the time he was nine. "I spent a lot of time as a boy playing in Central Park and walking around Manhattan by myself," he recalled in a 1981 Boston Globe interview. He was raised by an older brother and the family's West Indian housekeeper, who taught him to cook, which later came in handy when his wife Anne Bernays turned out to be a self-described "domestic illiterate".

A top student, Kaplan entered Harvard University at age 16, receiving a Bachelor's in English in 1944. After pursuing a post-graduate degree in English for two years, he grew dissatisfied with graduate school and moved to New Mexico. "The openness and the beauty of the Southwest," he said in the 1981 interview, "made me aware of American writers in a way I had never considered before."

He then began to work as an editor for the publishing house Simon & Schuster, where after eight years he rose to senior editor, becoming known as "the house brain", handling brainier authors including British philosopher Bertrand Russell, "Zorba the Greek" author Nikos Kazantzakis, and sociologist C. Wright Mills. Fascinated by words and language, by his early 20s Kaplan had edited translations of Plato and Aristotle. In his memoir Back Then (2002) Kaplan wrote: "It was fun to work at Simon & Schuster. [It was] not surprising to see editors staying long after hours to talk books, trade industry gossip, and joke over office bottles of Scotch and gin. In the days before it was absorbed into a conglomerate the house was like a summer camp for intellectually hyperactive children", only without a curfew, reminiscing about dancing at a party with Marilyn Monroe, "gently kneading the little tire of baby fat around her waist."

In 1953 while an editor at art book publisher Harry Abrams, he met Anne Bernays (b. 1930), daughter of public relations pioneer Edward L. Bernays and writer Doris E. Fleischman, and great-niece of Sigmund Freud. They married in 1954. Soon after he was invited by M. Lincoln "Max" Schuster, co-founder of Simon & Schuster to help acquire "better books", seek out younger authors, and "deal diplomatically" with established names.

In 1959 Kaplan saw Hal Holbrook's celebrated stage performance of Mark Twain, causing him to become fascinated with Twain, reading everything he could by and about him then writing a 10-page proposal complete with his own contract, which was accepted by Simon & Schuster complete with a $4,000 advance, causing him to leave publishing for writing, despite the anxiety caused by leaving a well-paying job for the uncertainty of a writer's life. Needing distance from the "adrenaline-intoxicated style" of New York, and needing access to Harvard's Widener Library, he and Anne moved to Massachusetts, where he remained for the rest of his life, living in Cambridge, Massachusetts in a 16-room house on Francis Avenue, where "Anne and Joe" became the center of a literary social circle at the heart of 02138, the Harvard Square ZIP code, with neighbors including French chef Julia Child and Harvard economist John Kenneth Galbraith. Said novelist James Carroll: "If there's a writer's community in Boston, they established it. There was a period of about 15 years when their house was the center of the writing life in Boston. Joe was the pillar, and Anne was the flame. Between the two of them they made a big difference in the life of the city."

In 1973 they built a home in Truro, Massachusetts in the Outer Cape.

Kaplan's first book Mr. Clemens and Mark Twain (1966) was a critical success, winning both the National Book Award in category Arts and Letters and the 1967 Pulitzer Prize for Biography or Autobiography. A stylish account of the Missouri-born humorist who attempted imperfectly to fit in with the Eastern elite, it was immediately praised as a landmark in Twain scholarship, making fans of E.L. Doctorow, Tom Wolfe et al. and becoming a standard biography. It “employed an organizing device, unusual for its day, to which Mr. Kaplan would return. Instead of arranging his subject’s life chronologically, he portrayed it out of sequence, opening the book with Twain at 31.”