Ursula K. Le Guin, the immensely popular author who brought literary depth and a tough-minded feminist sensibility to science fiction and fantasy with books like “The Left Hand of Darkness” and the Earthsea series, died on Monday at her home in Portland, Ore. She was 88.

Her son, Theo Downes-Le Guin, confirmed the death. He did not specify a cause but said she had been in poor health for several months.

Ms. Le Guin embraced the standard themes of her chosen genres: sorcery and dragons, spaceships and planetary conflict. But even when her protagonists are male, they avoid the macho posturing of so many science fiction and fantasy heroes. The conflicts they face are typically rooted in a clash of cultures and resolved more by conciliation and self-sacrifice than by swordplay or space battles.

Her books have been translated into more than 40 languages and have sold millions of copies worldwide. Several, including “The Left Hand of Darkness” — set on a planet where the customary gender distinctions do not apply — have been in print for almost 50 years. The critic Harold Bloom lauded Ms. Le Guin as “a superbly imaginative creator and major stylist” who “has raised fantasy into high literature for our time.”

In addition to more than 20 novels, she was the author of a dozen books of poetry, more than 100 short stories (collected in multiple volumes), seven collections of essays, 13 books for children and five volumes of translation, including the Tao Te Ching of Lao Tzu and selected poems by the Chilean Nobel Prize winner Gabriela Mistral. She also wrote a guide for writers.

Ms. Le Guin’s fictions range from young-adult adventures to wry philosophical fables. They combine compelling stories, rigorous narrative logic and a lean but lyrical style to draw readers into what she called the “inner lands” of the imagination. Such writing, she believed, could be a moral force.

“If you cannot or will not imagine the results of your actions, there’s no way you can act morally or responsibly,” she told The Guardian in an interview in 2005. “Little kids can’t do it; babies are morally monsters — completely greedy. Their imagination has to be trained into foresight and empathy.”

The writer’s “pleasant duty,” she said, is to ply the reader’s imagination with “the best and purest nourishment that it can absorb.”

She was born Ursula Kroeber in Berkeley, Calif., on Oct. 21, 1929, the youngest of four children and the only daughter of two anthropologists, Alfred L. Kroeber and Theodora Quinn Kroeber. Her father was an expert on the Native Americans of California, and her mother wrote an acclaimed book, “Ishi in Two Worlds” (1960), about the life and death of California’s “last wild Indian.”

At a young age, Ms. Le Guin immersed herself in books about mythology, among them James Frazier’s “The Golden Bough,” classic fantasies like Lord Dunsany’s “A Dreamer’s Tales,” and the science-fiction magazines of the day. But in early adolescence she lost interest in science fiction, because, she recalled, the stories “seemed to be all about hardware and soldiers: White men go forth and conquer the universe.”

She graduated from Radcliffe College in 1951, earned a master’s degree in romance literature of the Middle Ages and Renaissance from Columbia University in 1952, and won a Fulbright fellowship to study in Paris. There she met and married another Fulbright scholar, Charles Le Guin, who survives her.

On their return to the United States, she abandoned her graduate studies to raise a family; the Le Guins eventually settled in Portland, where Mr. Le Guin taught history at Portland State University.

Besides her husband and son, Ms. Le Guin is survived by two daughters, Caroline and Elisabeth Le Guin; two brothers, Theodore and Clifton Kroeber; and four grandchildren.

By the early 1960s Ms. Le Guin had written five unpublished novels, mostly set in an imaginary Central European country called Orsinia. Eager to find a more welcoming market, she decided to try her hand at genre fiction.

Her first science-fiction novel, “Rocannon’s World,” came out in 1966. Two years later she published “A Wizard of Earthsea,” the first in a series about a made-up world where the practice of magic is as precise as any science, and as morally ambiguous.

The first three Earthsea books — the other two were “The Tombs of Atuan” (1971) and “The Farthest Shore” (1972) — were written, at the request of her publisher, for young adults. But their grand scale and elevated style betray no trace of writing down to an audience.

The magic of Earthsea is language-driven: Wizards gain power over people and things by knowing their “true names.” Ms. Le Guin took this discipline seriously in naming her own characters. “I must find the right name or I cannot get on with the story,” she said. “I cannot write the story if the name is wrong.”

The Earthsea series was clearly influenced by J. R. R. Tolkien’s “The Lord of the Rings” trilogy. But instead of a holy war between Good and Evil, Ms. Le Guin’s stories are organized around a search for “balance” among competing forces — a concept she adapted from her lifelong study of Taoist texts.

She returned to Earthsea later in her career, extending and deepening the trilogy with books like “Tehanu” (1990) and “The Other Wind” (2001), written for a general audience.

“The Left Hand of Darkness,” published in 1969, takes place on a planet called Gethen, where people are neither male nor female but assume the attributes of either sex during brief periods of reproductive fervor. Speaking with an anthropological dispassion, Ms. Le Guin later referred to her novel as a “thought experiment” designed to explore the nature of human societies.

“I eliminated gender to find out what was left,” she told The Guardian.

But there is nothing dispassionate about the relationship at the core of the book, between an androgynous native of Gethen and a human male from Earth. The book won the two major prizes in science fiction, the Hugo and Nebula awards, and is widely taught in secondary schools and colleges.

Much of Ms. Le Guin’s science fiction has a common background: a loosely knit confederation of worlds known as the Ekumen. This was founded by an ancient people who seeded humans on habitable planets throughout the galaxy — including Gethen, Earth and the twin worlds of her most ambitious novel, “The Dispossessed,” subtitled “An Ambiguous Utopia” (1974).

As the subtitle implies, “The Dispossessed” contrasts two forms of social organization: a messy but vibrant capitalist society, which oppresses its underclass, and a classless “utopia” (partly based on the ideas of the Russian anarchist Peter Kropotkin), which turns out to be oppressive in its own conformist way. Ms. Le Guin leaves it up to the reader to find a comfortable balance between the two.

“The Lathe of Heaven” (1971) offers a very different take on utopian ambitions. A man whose dreams can alter reality falls under the sway of a psychiatrist, who usurps this power to conjure his own vision of a perfect world, with unfortunate results.

“The Lathe of Heaven” was among the few books by Ms. Le Guin that have been adapted for film or television. There were two made-for-television versions, one on PBS in 1980 and the other on the A&E cable channel in 2002.

Among the other adaptations of her work were the 2006 Japanese animated feature “Tales From Earthsea” and a 2004 mini-series on the Sci Fi channel, “Legend of Earthsea.”

With the exception of the 1980 “Lathe of Heaven,” she had little good to say about any of them.

Ms. Le Guin always considered herself a feminist, even when genre conventions led her to center her books on male heroes. Her later works, like the additions to the Earthsea series and such Ekumen tales as “Four Ways to Forgiveness” (1995) and “The Telling” (2000), are mostly told from a female point of view.

In some of her later books, she gave in to a tendency toward didacticism, as if she were losing patience with humanity for not learning the hard lessons — about the need for balance and compassion — that her best work so astutely embodies.

At the 2014 National Book Awards, Ms. Le Guin was given the Medal for Distinguished Contribution to American Letters. She accepted the medal on behalf of her fellow writers of fantasy and science fiction, who, she said, had been “excluded from literature for so long” while literary honors went to the “so-called realists.”

She also urged publishers and writers not to put too much emphasis on profits.

“I have had a long career and a good one,” she said, adding, “Here at the end of it, I really don’t want to watch American literature get sold down the river.”