Five Chinese nationals, including two children, are suspected of using a small inflatable boat to illegally enter Guam.
On Tuesday, a Guam National Wildlife Refuge park ranger spotted a two-year-old, a 14-year-old, their parents and another man in an area of the refuge, which includes Ritidian beach, a popular tourist haven, the Pacific Daily News reported.
They were wet and carrying several bags and fishing rods, the park service said.
When the ranger then found their inflatable boat, she called immigration authorities and the family group was taken into custody.
United States immigration officers said the yellow inflatable boat, powered by an outboard motor, came from the Northern Mariana Islands.
Chinese nationals are able to travel to the Commonwealth of the Northern Mariana Islands through a visa-waiver programme.
Since the late 1990s, the 9,100-hectare wildlife refuge has been used often as a beach entry point for illegal immigrants, mostly from China.
Some have come from the garment factories in the Northern Marianas that used mostly Chinese workers.
In 1999, Guam's detention facility was overwhelmed by attempts to enter the island on boats.
The adults were detained, but immigration officers said the mother would be released to care for the children, while her deportation case was decided in court.
All five appeared in good health after surviving the voyage over rough seas.
The two men suffered from minor sunburn, said Virginia Kice, the immigration western regional communications director.
US security special agents were investigating, Kice said.