Gen. Lewis W. Walt, who won combat decorations in World War II, Korea and Vietnam and later served as assistant commandant of the Marine Corps, died Sunday at a retirement home in Gulfport, Miss., after a long illness. He was 76 years old.

General Walt, a husky, outspoken man who commanded 73,000 marines in the critical I Corps area in the five northernmost provinces of South Vietnam from mid-1965 to mid-1967, had a reputation of being a front-line general. A Vietcong mortar round once exploded within 15 feet of him, wounding his aide. On another occasion he walked across a mined bridge to confront a group of dissident South Vietnamese troops while Marine and Navy frogmen were cutting the wires leading to the explosive charges.

General Walt was also a tireless advocate of what he called ''the other war'' in Vietnam: winning the hearts and minds of Vietnamese civilians. One year the marines under his command distributed 2.5 million pounds of food and 237,000 pounds of clothing, and built 673 houses and 159 classrooms.

General Walt, who wrote three books after retiring from the Marine Corps, described the contradictions of the Vietnam War in an article for The New York Times in 1971. ''On the one hand it was an extremely sophisticated war, with complex weapons unlike even World War II or Korea,'' he wrote. ''On the other hand it was a return to medieval war, pitting man against man on a battleground where only the courageous could win.'' News Media Role Cited

Like many American military officers, General Walt was frustrated by what he regarded as a lack of public support for the nation's troops in Vietnam, calling it the ''most misundertood war in our country's history.'' And, like many officers, he criticized press and television coverage of the war.

But he rejected censorship and anticipated a point of view that is slowly spreading among officers of the post-Vietnam generation who are now students in the nation's war colleges. ''We are going to have to realize the supreme importance of the news media in fighting political wars in the future,'' he said in 1969, ''or we will be faced otherwise with unpopular wars wherever they are.''

Lewis William Walt was born on a farm near Harveyville, Kan., on Feb. 16, 1913. He received a Bachelor of Science degree, with honors in chemistry, from Colorado State University, where he was a star football player. He was commissioned as a second lieutenant in the Marine Corps on July 1, 1936.

The future general served as a platoon leader in Shanghai, China, guarding the international settlement there after Japan invaded northern China. Earned Two Navy Crosses

In World War II he took part in the first American ground offensive in the Pacific theater, leading a company of the First Marine Raider Battalion that landed on Tulagi in the Solomon Islands in August 1942. He was awarded the Silver Star for gallantry in that landing, and later joined the Fifth Marine Regiment on Guadacanal, where he was wounded in action and promoted to major and then lieutenant colonel.

In 1944 he commanded the Third Battalion of the Fifth Marines in the campaign for New Britain, where he won the Navy Cross, the nation's second highest combat decoration, and was again wounded in action. After recovering from his wounds he resumed command of the battalion and won his second Navy Cross in the landing on Peleliu, one of the Caroline Islands in the western Pacific.

In the Korean War he was promoted to colonel, served as a regimental commander in 1952 and later as chief of staff of the First Marine Division, and won the Bronze Star and Legion of Merit. He attended the National War College in 1960, was promoted to brigadier general and assigned as assistant commander of the Second Marine Division at Camp Lejuene, N. C., in 1962.

After being promoted to major general in 1965 he commanded the Third Marine Amphibious Force in Vietnam, was promoted to lieutenant general in 1966 and was awarded the Distinguished Service Medal. He later said that, as the Marine commander in the northern sector of South Vietnam, he had ''pleaded and begged to go into Laos and across the DMZ,'' the demilitarized zone that separated North and South Vietnam. A Contest for Commandant

Until 1968 the Marine Corps had only one four-star general, the Commandant. General Walt became the first officer in the Corps to become its second four-star general after losing a fight to become Commandant. He, Lieut. Gen. Victor H. Krulak and Lieut. Gen. Leonard F. Chapman Jr. were the contenders. General Chapman won out but General Walt got his fourth star as assistant commandant.

In a news conference just before he retired in 1971, General Walt criticized Congress, accusing it of following rather than leading the people. He also criticized parents for not teaching their children and local governments for not improving schools. ''We need to educate our children on why we need strong armed forces,'' he said. ''Neutrality is a great thing, but who is going to enforce neutrality?''

General Walt's three books were ''Strange War, Strange Strategy,'' about the war in Vietnam; ''America Faces Defeat,'' about the dangers confronting the nation, and ''The Eleventh Hour,'' about the urgency of the nation's problems.

The general's first marriage, to Nancy Mary Sheehan, an Army nurse he met in World War II, ended in divorce. He is survived by his second wife, June Burkett Jacobsen Walt, and two sons and a daughter by his first marriage, Lewis W. Walt Jr., Lawrence C. Walt and Mary K. Martin.

Lewis W Walt