Walter Matthau, whose performances as cantankerous but endearing characters made him a distinctive leading man in movies, theater and television, died yesterday in Santa Monica, Calif. He was 79.

The cause was a heart attack, said Lindi Funston, a spokeswoman for St. John's Health Center, where he died.

Mr. Matthau's breakthrough role was as Oscar Madison, the slovenly sportswriter in Neil Simon's 1965 Broadway comedy and 1968 film ''The Odd Couple.'' But that was only one of an extraordinarily diverse galaxy of characters. In his long career he played lawyers, editors, detectives, spies and sheriffs, and each one of them looked and acted like Walter Matthau.

In 1994, he played a comic version of Albert Einstein in the movie ''I.Q.,'' and Einstein began to resemble the shaggy, shambling actor. Although Mr. Matthau was an experienced stage actor, in common with great film stars of the past like Spencer Tracy and Humphrey Bogart, he created character through the magnetism of his own personality.

Mr. Simon credited Mr. Matthau's rumpled persona and ironic wit as being the inspiration for Oscar, the nemesis of his fussy apartment-mate and fellow dropout from marriage, Felix Unger (Art Carney on stage, Jack Lemmon in the film). In his memoirs, the playwright said the actor at first wanted to play Felix, in retrospect a mind-boggling idea, but one that Mr. Matthau undoubtedly could have accomplished. He was, Mr. Simon said, ''the greatest instinctive actor I've ever seen.''

Mr. Matthau was a tall man with a perpetual slouch and a perpetual frown. No matter how elegant his character was supposed to be, in common with Oscar he usually looked as if he had been sleeping in his clothes. But through a kind of reverse chic Mr. Matthau could make himself seem stylish. Without airs or affectations, he became a widely respected international star.

Though he often played dramatic roles, comedy was his principal metier. With his hangdog face and growling voice, he added humor to his line readings. Dialogue that would not be so funny in print became hilarious when delivered by Mr. Matthau. With perfect timing, he was an expert at the slow burn, the double take, the explosion into exasperation. ''Walter doesn't need funny lines,'' George Burns, who starred with him in the movie of Mr. Simon's play ''The Sunshine Boys,'' once said. ''He's fearless -- no inhibitions. If you want him to play soprano, or be a toe dancer, he'd do it.''

A Comedy Duo Through the Years

It was with Jack Lemmon that he became half of one of America's favorite, if floating, comedy teams. In addition to ''The Odd Couple,'' they were in three Billy Wilder comedies, ''The Fortune Cookie,'' ''The Front Page'' and ''Buddy Buddy.'' The two aged together before the camera until they were playing the twinned title roles in ''Grumpy Old Men.'' In the sequel, they became ''Grumpier Old Men,'' and probably would have been on their way to ''Grumpiest.''

In a statement yesterday from Los Angeles, Mr. Lemmon said: ''I have lost someone I loved as a brother, as a closest friend and a remarkable human being. We have also lost one of the best damn actors we'll ever see.''

Last year in the book ''Conversations With Wilder,'' Mr. Wilder said that Mr. Matthau was ''one of the handful of great actors.''

Mr. Matthau worked his way up from poverty on the Lower East Side of Manhattan to varied roles in stock, more than 20 Broadway plays (mostly flops) and scores of live television shows. He became, successively, an adept movie villain and comedian and later, improbably, a romantic leading man.

He was tireless in improvising fresh comic business to sharpen and enhance plays and films, and in spinning perceptive observations from commonplace events. With sly self-mockery, he described his Hollywood image as a ''Ukrainian Cary Grant.''

Film portraits shaped by Mr. Matthau included a tricky lawyer in ''The Fortune Cookie'' (a performance that earned him a 1966 Academy Award as best supporting actor), an inept rake in ''A Guide for the Married Man'' (1967), sly philanderers in ''The Secret Life of an American Wife'' (1968) and ''Cactus Flower'' (1969), and three tour de force roles in Mr. Simon's ''Plaza Suite'' (1971), as a callous husband, a vulgar filmmaker and a put-upon father.

He enlivened thrillers like Don Siegel's ''Charley Varrick'' (1973) and ''The Taking of Pelham One Two Three'' (1974), and he played W. C. Fields-like child-detesters in ''The Bad News Bears'' (1976) and the 1980 remake of ''Little Miss Marker.'' Other characters were a lugubrious editor in ''The Front Page'' (1974), a henpecked assassin in ''Buddy Buddy'' (1981) and a crusty Supreme Court justice in ''First Monday in October'' (1981). In 1993 he was the beleaguered Mr. Wilson in ''Dennis the Menace.''

Mr. Matthau's personality also gleamed in bouts with other potent personalities: Barbra Streisand in ''Hello, Dolly!'' (1969), Elaine May in ''A New Leaf'' (1971), Carol Burnett in ''Pete 'n' Tillie'' (1972), Mr. Burns in ''The Sunshine Boys'' (1975), and Glenda Jackson in ''House Calls'' (1978) and ''Hopscotch'' (1980).

Comedy, he said in 1981, should be shaped very seriously and have an emotional foundation, evoke genuine laughter and then provide some insight. Asked to compare stage and film acting, he said: ''Theater is pure, while movies are bits and pieces. Right now, I like acting in movies better because it's much easier, more fun, though not as satisfying. Doing a play is like having a seven-course meal, but a movie is like eating a lot of hors d'oeuvres. You get filled up, but you're never quite satisfied.

''There are very few good stage actors,'' he continued, ''but there are many good movie actors,'' partly because of efforts by the director, the cameraman, the editor. ''On the stage, there are no tricks, it's you.''

Walter Matthau was born in New York City on Oct. 1, 1920, to Milton Matuschanskayasky and the former Rose Berolsky, impoverished Jewish immigrants. His father, from Russia, was an electrician turned process server who abandoned the family when Walter was 3 years old and who died 12 years later. Walter's mother, who was from Lithuania, kept him and his elder brother, Henry, in a day nursery while she worked as a sweatshop seamstress. The family moved often because she could not pay the rent for their cold-water flats.

Acting Offers a Refuge For a Boy in Poverty

Young Walter took refuge in acting, appearing in theatricals in schools and settlement houses and reading Shakespeare nearly every day. At 11, he started selling ice cream and soft drinks in the Yiddish theaters on Second Avenue and, for an extra 50 cents a show, did bit parts while studying the great Yiddish actors. At Seward Park High School, he lettered in six sports and was also in demand as a campaign manager for student office-seekers, he once said, because schoolmates were invariably won over by ''the way I stood or looked or sounded.''

Jobs after graduation included forestry in Montana in the Civilian Conservation Corps and coaching boxing and basketball in New York City. In World War II, he enlisted in the Army Air Corps. He served as a radio operator and cryptographer in Europe, rose to staff sergeant and earned six battle stars.

Back in Manhattan, he used his G.I. benefits to study at the New School's Dramatic Workshop under the director, Erwin Piscator. He later recounted the following exchange with Mr. Piscator:

'' 'Matthau, you say here every line wrong.' Another actor might have been destroyed. I was destroyed for about 20 seconds. Then I said, 'Teach me how to say every line right.' One way and another, I began to learn how to talk, how to look, how to walk -- how to act.''

In a summer stock production of ''Three Men on a Horse'' in 1946, he received what he considered to be the finest compliment. A theatergoer told him he was the only one in the play he did not like. ''The others looked like actors,'' he said. ''You just looked like a poolroom bum.''

The irrepressible Mr. Matthau was reflective and personable in interviews, but he sometimes enlivened them with outrageous lies, saying that his father had been a renegade, defrocked Orthodox priest, and falsifying his training credentials, saying he had studied at Oxford University, the Abbey Theater in Dublin and the Moscow Art Theater.

On Broadway, he advanced from a candelabra carrier in ''Anne of the Thousand Days'' (in 1948) to a bookish lover in ''The Ladies of the Corridor,'' an urbane playwright in ''Will Success Spoil Rock Hunter?'' and an exuberant impresario in ''Once More With Feeling.'' In 1962 he won a Tony award for his performance as a Parisian aristocrat in ''A Shot in the Dark,'' and three years later won another Tony for ''The Odd Couple.''

In acting, he used his intuition and his native intelligence about people, especially those with eccentricities. ''You study the character by living with him,'' he said. ''Even when you're sleeping, you're developing the character. An actor shouldn't think on the stage. He must only do.''

A Reformed Man After a Heart Scare

In 1966 Mr. Matthau suffered a severe heart attack brought on, his doctor said, by heavy smoking, compulsive gambling and insufficient exercise. The crisis created a reformed man: a nonsmoker, a more moderate gambler (or so it was said) and a dedicated walker.

His obsession with gambling reached a low point early in his career when he bet and lost $183,000 in only two weeks on spring-training baseball games and scrimped for six years to pay off the debt to an underworld loan shark. But his tendency to take risks paid off later, when he reaped a fortune after opting to receive percentages of film profits rather than salaries. He later still placed bets, but relatively small ones, on racehorses that he owned with friends.

Mr. Matthau disliked jogging but became an avid walker, averaging 15 miles a week and varying his pace along ocean beaches near his home in Pacific Palisades, Calif. ''It makes you ease up, if you have any sense,'' he said of the heart attack. ''It makes you think about what's important. I feel as though I've been given a second life, one better than the old one.'' In 1976, Mr. Matthau underwent heart bypass surgery. Last year, he was hospitalized for two months with pneumonia.

He was a lifelong enthusiast of sports and classical music, habitually playing records at home and tapes in his cars. In contrast to the crotchety characters he generally played on screen, he was known to friends and colleagues as one of the best natured of men. In later years, he made about one film a year, and when an interviewer suggested in 1990 that he might be approaching retirement, Mr. Matthau replied sharply: ''That word is alien to my vocabulary. Retire to what? I'm doing the work I like.''

He also said he would not direct another film. His first and last was a 1960 low-budget melodrama, ''Gangster Story.'' ''It was one of the worst films ever made,'' he said with characteristic candor, and then added, ''Let me put it another way: it was the worst I ever made.''

His son Charles eventually became a movie director. In 1996 he made a film of ''The Grass Harp,'' in which Walter Matthau played gentlemanly Judge Cool and Jack Lemmon was a salesman.

Mr. Matthau, who teamed up with Mr. Lemmon in ''The Odd Couple II'' in 1998, most recently appeared in ''Hanging Up,'' released in February and starring Diane Keaton, Meg Ryan and Lisa Kudrow.

Charles Matthau said that his father approved his choice of career although he suggested that it would have been wiser for his son to have a newsstand and ''find a husky woman who, at the end of a day's work, can carry me home.''

In addition to Charles, Mr. Matthau is survived by his wife, Carol, and two children from a previous marriage, another son, David, and a daughter, Jenny.

Also in 1996, Mr. Matthau starred opposite Ossie Davis in the film version of Herb Gardner's play ''I'm Not Rappaport.'' The role had been created on Broadway by Judd Hirsch and had been played in London by Paul Scofield, but, as was often the case, once Mr. Matthau played a character, he made it definitively his own. Not only did the role seem written specifically for the actor, it also seemed as if he might have been the real-life model: a stubborn old codger who refused to surrender his independence, an irascible (but lovable) survivor of all wars, political and domestic.

Matthau on Film

These are among Walter Matthau's most important films:

A Face in the Crowd (1957)

The Fortune Cookie (1966)

A Guide for the Married Man (1967)

The Odd Couple (1968)

Hello, Dolly! (1969)

Plaza Suite (1971)

A New Leaf (1971)

The Sunshine Boys (1975)

The Bad News Bears (1976)

Casey's Shadow (1978)

Hopscotch (1980)

Grumpy Old Men (1993)

I'm Not Rappaport (1996)