Monica Vitti, whose chilly sensuality and cerebral approach to her roles enlivened a groundbreaking series of 1960s film masterpieces directed by Michelangelo Antonioni, including the much-debated “L’Avventura,” died on Wednesday in Rome. She was 90.

Her death was announced by the filmmaker Walter Veltroni, a former mayor of Rome and culture minister of Italy.

In a news release, Dario Franceschini, Italy’s culture minister, wrote, “Goodbye to the queen of Italian cinema.”

A classically trained actress, Ms. Vitti was already an established stage star in Italy in 1957 when she met Antonioni, who subsequently became her companion for a decade, just as she became his muse and alter ego.

Ms. Vitti emerged on the international scene as all eyes were turning to Europe, where a new generation of visionary filmmakers was remaking the landscape, particularly in France and Italy. Her sharp, patrician features and icy demeanor provided a visual and stylistic counterpoint to the working-class voluptuousness of the leading Italian actresses of the period, among them Sophia Loren and Anna Magnani.

The official screening of “L’Avventura” at the 1960 Cannes International Film Festival became a milestone in film history. It ended with a chorus of boos from the audience, perplexed by a film that began as a mystery about a missing woman named Anna and morphed into a nearly emotionless sexual interlude between the missing woman’s fiancé and her best friend, played by Ms. Vitti.

Antonioni thought his career was over. Ms. Vitti fled the auditorium in tears when her most heartfelt scenes were greeted with laughter. But a cabal of filmmakers, led by Roberto Rossellini, wrote an impassioned defense of the film, and it went on to win the festival’s Special Jury Prize and to become widely hailed as a cinematic landmark.

Mainstream critics were split on the film, which drew as much puzzlement as it did praise. Pauline Kael of The New Yorker declared it the best film of the year and praised Ms. Vitti’s work. In his New York Times review, Bosley Crowther panned the film and described her performance as “weirdly coquettish and intense.” But it made her into an instant international star, and in 1962 the British film magazine Sight & Sound declared it the second-best movie ever made, after “Citizen Kane.”

She also starred in Antonioni’s first color film, “Red Desert” (1964), which many critics described as being like a fourth film in the alienation series.

Neorealism, which had dominated Italian cinema since the end of World War II, was being superseded in the late 1950s and early ’60s by fresh approaches. Federico Fellini became a global figure on the strength of a series of exuberant films like “La Strada” and “Nights of Cabiria.”

Ms. Vitti’s breakout role in “L’Avventura” came at about the same time Fellini unveiled his most influential film to that point, “La Dolce Vita.” The two films shared a pessimism about modern life, but otherwise could not have been more different. Fellini’s film embraced audiences with its seductiveness, while Antonioni’s was maddeningly obscure, not so much failing to meet audience expectations as ostentatiously ignoring them.

A romantic relationship blossomed between Ms. Vitti and Antonioni during the filming of “L’Avventura” and grew stronger in the years that followed. At one point, before their relationship became widely known, Ms. Vitti lived in an apartment just below Antonioni’s in Rome, and the director had a trap door and spiral staircase installed so they could see each other whenever they liked without rousing outside notice.

An attempt to transform her into a mainstream star in the psychedelic British spy satire “Modesty Blaise” in 1966 fell flat despite a strong cast, including Terence Stamp and Dirk Bogarde, and despite having the acclaimed director Joseph Losey in command.

After her relationship with Antonioni ended in 1967 and she stopped making movies with him, she decided to reinvent her entire career, switching to light comedies, which at the time in Italy were dominated by male stars. Italian audiences and critics were stunned by her facility as a comedian, which many came to believe was her greatest calling.

She continued to be a beloved star in Italy, though few of her films from these years, which had titles like “Kill Me Quick, I’m Cold” and “The Girl With a Pistol,” found an international audience. One exception was Ettore Scola’s “Dramma della Gelosia,” released in the United States in 1970 as “The Pizza Triangle,” which was a substantial hit.

In 1974, she worked with another celebrated filmmaker, Luis Buñuel, in “The Phantom of Liberty,” her last great critical success.

Monica Vitti was born Maria Luisa Ceciarelli in Rome on Nov. 3, 1931, the third child and only daughter of Angelo and Adele (Vittilia) Ceciarelli. She shortened her mother’s maiden name to use as her own stage name.

She later recalled a troubled and poverty-stricken childhood under strict parents who kept her secluded at home while allowing her brothers the freedom of the city, which she bitterly resented. The experience, she said, made her wary of marriage and disinclined to have children.

In part to escape her unhappy home, she began acting as a teenager.

When she was 18, her parents and brothers emigrated to the United States seeking a change in their fortunes, but she remained in Rome, where she graduated from the National Academy of Dramatic Art in 1953.

“I used their absence to become an actress,” she said. “When they came back, my parents had to call me Monica. They had to acknowledge what had happened.”

She had film roles as early as 1954, but was best known in this period as a stage and television actress. She met Antonioni in 1957, but he was having difficulty raising money to make movies at that time, and it was not until “L’Avventura” that he was able to showcase his new star and paramour.

In the wake of the Antonioni trilogy, Ms. Vitti became one of the more glamorous figures on the international film scene, a regular fixture at Cannes and other international events. Her celebrity continued, in Europe at least, after she had made the switch to light comedies in the 1970s.

She struck up a relationship in 1975 with another Italian filmmaker, Roberto Russo, a cameraman, screenwriter and director. They lived together for many years before finally marrying in 1995. He survives her.

In 1979 Ms. Vitti was recruited by the Hollywood director Michael Ritchie for “An Almost Perfect Affair,” in which she played the wife of an Italian movie mogul, who strikes up a romance at the Cannes festival with a young filmmaker played by Keith Carradine.

Ms. Vitti worked less frequently onscreen in the 1980s and returned her attention to stage work. She also taught acting. In 1989 she tried her hand at directing with “Secret Scandal,” a film she also wrote and in which she starred with Elliott Gould. It drew praise, but it failed at the box office and marked the end of her big-screen career.

For the next decade, she worked infrequently in Italian television.

During the height of her fame, after the release of “L’Avventura,” Ms. Vitti usually left it to Antonioni to try to explain to journalists what he meant by the film — especially why he refused to resolve the mystery of Anna’s disappearance.

But in a New York Times interview in Manhattan just days before the film’s New York release, Ms. Vitti gave it a try, with an answer almost as enigmatic as the film.

“That’s the one question the audience isn’t supposed to ask,” she explained. “It isn’t important. What is important is that Anna was carrying two books before she disappeared — the Bible and F. Scott Fitzgerald’s ‘Tender Is the Night.’ One suggests our concern with morality; the other was a literary experiment in which the heroine disappears halfway through the book and is replaced by another protagonist.”

Alex Marshall and Elisabetta Povoledo contributed reporting.