First released in Italy on 14 February 1963, Otto e mezzo received virtually unanimous acclaim, with reviewers hailing Fellini as "a genius possessed of a magic touch, a prodigious style". Italian novelist and critic Alberto Moravia described the film's protagonist, Guido Anselmi, as "obsessed by eroticism, a sadist, a masochist, a self-mythologizer, an adulterer, a clown, a liar and a cheat. He's afraid of life and wants to return to his mother's womb.... In some respects he resembles Leopold Bloom, the hero of James Joyce's Ulysses, and we have the impression that Fellini has read and contemplated this book. The film is introverted, a sort of private monologue interspersed with glimpses of reality.... Fellini's dreams are always surprising and, in a figurative sense, original, but his memories are pervaded by a deeper, more delicate sentiment. This is why the two episodes concerning the hero's childhood at the old country house in Romagna and his meeting with the woman on the beach in Rimini are the best of the film, and among the best of all Fellini's works to date".
Reviewing for Corriere della Sera, Giovanni Grazzini underlined that "the beauty of the film lies in its 'confusion'... a mixture of error and truth, reality and dream, stylistic and human values, and in the complete harmony between Fellini's cinematographic language and Guido's rambling imagination. It is impossible to distinguish Fellini from his fictional director and so Fellini's faults coincide with Guido's spiritual doubts. The osmosis between art and life is amazing. It will be difficult to repeat this achievement. Fellini's genius shines in everything here, as it has rarely shone in the movies. There isn't a set, a character or a situation that doesn't have a precise meaning on the great stage that is 8½". Mario Verdone of Bianco e Nero insisted the film was "like a brilliant improvisation.... The film became the most difficult feat the director ever tried to pull off. It is like a series of acrobats [sic] that a tightrope walker tries to execute high above the crowd,... always on the verge of falling and being smashed on the ground. But at just the right moment, the acrobat knows how to perform the right somersault: with a push he straightens up, saves himself and wins".
8½ screened at the 1963 Cannes Film Festival in April to "almost universal acclaim" and was Italy's official entry in the later 3rd Moscow International Film Festival where it won the Grand Prize. French film director François Truffaut wrote: "Fellini's film is complete, simple, beautiful, honest, like the one Guido wants to make in 8½". Premier Plan critics André Bouissy and Raymond Borde argued that the film "has the importance, magnitude, and technical mastery of Citizen Kane. It has aged twenty years of the avant-garde in one fell swoop because it both integrates and surpasses all the discoveries of experimental cinema". Pierre Kast of Les Cahiers du Cinéma explained that "my admiration for Fellini is not without limits. For instance, I did not enjoy La strada but I did I vitelloni. But I think we must all admit that 8½, leaving aside for the moment all prejudice and reserve, is prodigious. Fantastic liberality, a total absence of precaution and hypocrisy, absolute dispassionate sincerity, artistic and financial courage – these are the characteristics of this incredible undertaking".
Released in the United States on 25 June 1963 by Joseph E. Levine, who had bought the rights sight unseen, the film was screened at the Festival Theatre in New York in the presence of Fellini and Marcello Mastroianni. The acclaim was unanimous with the exception of reviews by Judith Crist, Pauline Kael, and John Simon. Crist "didn't think the film touched the heart or moved the spirit". Kael derided the film as a "structural disaster" while Simon considered it "a disheartening fiasco". Newsweek defended the film as "beyond doubt, a work of art of the first magnitude". Bosley Crowther praised it in the New York Times as "a piece of entertainment that will really make you sit up straight and think, a movie endowed with the challenge of a fascinating intellectual game.... If Mr. Fellini has not produced another masterpiece – another all-powerful exposure of Italy's ironic sweet life – he has made a stimulating contemplation of what might be called, with equal irony, a sweet guy". Archer Winsten of The New York Post interpreted the film as "a kind of review and summary of Fellini's picture-making" but doubted that it would appeal as directly to the American public as La Dolce Vita had three years earlier: "This is a subtler, more imaginative, less sensational piece of work. There will be more people here who consider it confused and confusing. And when they do understand what it is about – the simultaneous creation of a work of art, a philosophy of living together in happiness, and the imposition of each upon the other, they will not be as pleased as if they had attended the exposition of an international scandal". Audiences, however, loved it to such an extent that a company attempted to obtain the rights to mass-produce Guido Anselmi's black director's hat.
Fellini biographer Hollis Alpert noted that in the months following its release, critical commentary on 8½ proliferated as the film "became an intellectual cud to chew on". Philosopher and social critic Dwight Macdonald, for example, insisted it was "the most brilliant, varied, and entertaining movie since Citizen Kane". In 1987, a group of thirty European intellectuals and filmmakers voted Otto e mezzo the most important European film ever made. In 1993, Chicago Sun-Times film reviewer Roger Ebert wrote that "despite the efforts of several other filmmakers to make their own versions of the same story, it remains the definitive film about director's block". It came number two on the 1992 and 2002 Sight & Sound Director's Poll beaten only by Citizen Kane. 8½ is a fixture on the British Film Institute's Sight & Sound critics' and directors' polls of the top ten films ever made. It ranked number two on the magazine's 2002 Directors' Top Ten Poll and number eight on the Critics' Top Ten Poll. and stayed within the top ten, but slightly lower in the 2012 poll (number four on the 2012 directors' poll and ten on the 2012 critics' poll). Director Martin Scorsese also listed it as one of his favourite films of all time.