Dennis Lloyd Martin (born June 20, 1962) was an American child who disappeared on June 14, 1969, in the Great Smoky Mountains National Park in Tennessee at the age of six. The search effort was the most extensive in the park's history, involving approximately 1,400 searchers and a 56-square-mile (150 km2) area.
 Dennis Martin, a six-year-old resident of Knoxville, was visiting the Great Smoky Mountains National Park along with his father, grandfather and older brother on Father's Day weekend in 1969. The camping trip was a family tradition for the Martins. The family hiked from Cades Cove to Russell Field and camped overnight. The next day, they hiked to Spence Field near the Appalachian Trail, where they planned to spend the night.
 Martin disappeared on June 14 at 16:30 while planning on surprising the adults with his brother and other children from a separate family the Martins were camping with; he was last seen by his father going behind a bush to hide, intending on surprising the adults with the other children. After not seeing him for about five minutes and when all of the other children had returned to the camp site, his father became concerned and began searching for him. His father ran down the trail for nearly two miles, until he was sure he could not have gotten any farther. After several hours, they sought help from National Park Service rangers.
 The area where Martin disappeared is marked by steep slopes and ravines. Wild animals such as copperhead snakes, bears, feral hogs, and bobcats inhabit the area. A downpour broke out shortly after Martin's disappearance, dropping 3 inches (7.6 cm) of rain in a matter of hours, which washed out trails and caused streams to flood. Temperatures on the night of June 14 dropped to nearly 50 °F (10 °C).
 Search efforts, including a separate search by the National Guard and Green Berets found no trace. Heavy rains during the first day's search and heavy mist the next day's hampered efforts. Up to 1,400 people were involved in the search effort, potentially obscuring possible clues. Footprints were found in the area, but were dismissed from the possibility of being Martin's by park officials, who determined them to have been left instead by a Boy Scout participating in the search. The child-sized footprints led to a stream, where they disappeared. The tracks indicated that one foot was barefoot, while the other was in an Oxford (the type of shoe Martin was wearing) or a tennis shoe. Retired park ranger and author Dwight McCarter believes that the prints likely belonged to Martin, as the tracks were not part of a group and none of the Boy Scouts were searching while barefoot.
 By June 22, 56 square miles (150 km2) of ground had been covered. More than a thousand searchers continued to look until June 26, when the search was cut back. The search was abandoned on June 29, after a last search.
The search was officially closed down on September 14, 1969. As of 2022, it is still the largest search in the history of Great Smoky Mountains National Park.
 Dennis's father offered a US$5,000 (equivalent to $39,900 in 2022) reward for information. Psychics, including Jeane Dixon, offered clues, but nothing was found. A few years after, a ginseng-hunter claimed to have discovered the scattered skeletal remains of a small child in Big Hollow, Tremont. He kept the find to himself until 1985 because of fear that he would be prosecuted for the illegal ginseng. A subsequent search turned up nothing.
 The unsuccessful search for Martin led the National Park Service to review and amend its policies on searches for missing people.
 Three main theories exist about what happened to Martin.