Dana Andrews, the sturdy, square-jawed archetypal American hero of acclaimed films of the 1940's, including "The Best Years of Our Lives," "Laura" and "A Walk in the Sun," died on Thursday at Los Alamitos Medical Center in Orange County, Calif. He was 83 years old.

He had been hospitalized with pneumonia, a hospital spokeswoman said. In recent years Mr. Andrews lived at the John Douglas French Center for Alzheimer's Disease in Los Alamitos.

Although Mr. Andrews was never nominated for an Academy Award, his performances in important films won consistently high praise from critics. In Samuel Goldwyn's Oscar-winning production "The Best Years of Our Lives" (1946), Mr. Andrews played Fred Derry, a former Air Force bombardier facing an uncertain future after World War II.

That same year, Mr. Andrews played Sergeant Tyne in "A Walk in the Sun," adapted from the novel by Harry Brown. It concerned a few hours in the lives of a platoon of American infantrymen trying to capture a farmhouse in southern Italy held by the Germans. A Reviewing the film in The New York Times, Bosley Crowther called "A Walk in the Sun" one of the better films to come out of World War II and said Mr. Andrews was "most impressive" among a good cast.

Two years earlier, the actor won plaudits for his portrayal of the hard-boiled detective in "Laura" who was obsessed with the portrait of a woman (Gene Tierney) he thought had been murdered. Alton Cook, writing in The New York World-Telegram, said Mr. Andrews played the detective with "smoldering force." Trouble Getting Roles

With such credits, there were some who thought Mr. Andrews could look forward to a long career of substantial roles. But by the end of the 1950's, Mr. Andrews was having trouble obtaining roles he wanted. He had been type-cast as a youthful hero, and producers thought he was growing too old for that. They were also cutting back on the medium-budget films in which he had established himself.

"They want top box-office names for blockbusters," he told an interviewer late in the decade, "and I'm not in that category."

He briefly turned to the stage and found some work that suited him in 1958, when he replaced Henry Fonda in the Broadway production of "Two for the Seesaw." He told Don Cook of The New York Herald Tribune that he hoped the role would "be a showcase for me." Although he received good notices from critics, he never again was offered a role of the quality he had known in the 1940's, either in film or the theater.

Mr. Andrews had also developed a drinking problem. In 1957 he pleaded guilty to drunken driving and was fined $250 after his car hit a parked car in North Hollywood. The next year, the producer Benedict Bogeaus, and Waverly Productions Inc. sued him for $159,769 in damages, saying that a year earlier, on location for a movie in Mexico, Mr. Andrews's stay was "interrupted only by infrequent and occasional periods of sobriety." Head of Screen Actors Guild

Mr. Andrews continued to seek work and increasingly dedicated himself to the business of the Screen Actors Guild, in which he was a vice president and president. He worked to protect the wage scales of actors, and in 1963, after becoming president of the union, he spoke out on what he saw as the degradation of the profession.

He publicly criticized actresses who appeared nude in the movies and said women were being exploited in this way because greedy producers had decided they could make money "by having performers do something they cannot do on television." He warned that the day might come when actresses would feel under considerable pressure to work nude. If they refused, he predicted, they would either have to work in television or give up acting.

Television was not something Mr. Andrews especially liked, although he had television roles. From 1969 to 1972, as his career faded, he appeared in a daytime serial called "Bright Promise." But he attacked television as "just an adjunct of the advertising business." He did not approve of stars' doing commercials.

But in 1972 he made a commercial in which he said: "I'm Dana Andrews, and I'm an alcoholic. I don't drink anymore, but I used to -- all the time." In the commercial, which he made for the Federal Department of Transportation to educate people about the perils of drunken driving, he said he had not had a drink in four years. He added, "I'm a very happy man now, and I work all the time."

Not all the work was of the star caliber he knew when he was in his prime. Among his last films, made in the 1960's and 70's, were "In Harm's Way" and "Airport 1975."

Carver Dana Andrews was born on Jan. 1, 1909, in Collins, Miss., the third child among seven sons and two daughters born to the Rev. Charles Forrest Andrews, a Baptist minister, and the former Annis Speed. One of Dana's younger brothers was the actor Steve Forrest. The names Carver and Dana were bestowed because they were the last names of two professors under whom the minister had studied.

The family moved several times after Dana was born, settling in Huntsville, Tex. Dana graduated from Huntsville High School in 1926, enrolled in Sam Houston State Teachers College in Huntsville and majored in business administration. He played some football and left college after three years, becoming an accountant with the Gulf Oil Company in Austin. He developed an interest in acting, and in 1931 he hitchhiked to Los Angeles to see if he could get into the movies. He supported himself by working in a gasoline station in Van Nuys, Calif.

In 1938, he signed a contract with the Samuel Goldwyn studios. But Goldwyn had no work for him. In 1940, he appeared in "The Westerner," starring Gary Cooper. He had minor roles until 1943, when he landed one of the supporting roles in "Up in Arms," with Danny Kaye. That year, he was chosen to star in "The Purple Heart," a Hollywood account of fliers shot down during Maj. Gen. Jimmy Doolittle's raid on Tokyo. "Laura," the next year, was his first major triumph.

Among Mr. Andrews' other films were "The Ox-Bow Incident" (1943), "Wing and a Prayer" (1944), "Boomerang" (1947), "My Foolish Heart" (1950), "Zero Hour" (1957) and "The Last Tycoon" (1976).

Mr. Andrews married Janet Murray in 1932. She died in 1935. Their son, David, a musician and composer, died in 1964 after a cerebral hemorrhage. In 1939 Mr. Andrews married Mary Todd, from whom he was divorced in 1968. They had three children, Catherine, Susan and Stephen.