The park has lost about 26% of its 18,000 hectares (180 square kilometers) of forest cover since the United Nations World Food Program (WFP) started measuring it in 2016, according to the latest analysis.
The report warned that deforestation could contribute towards "slope destabilization, water shortages, landslides and floods" and called for the close monitoring of forest coverage to protect the city's main water sources.
The city of Freetown is built on a forested peninsula, which is mostly made up of national park land. Its water comes from reservoirs in the mountains, but deforestation is causing rain to drain off the hillsides rather than seeping through roots into the soil and streams.
"The issue of deforestation on the Western Area peninsula is tragic, worrying and alarming," said Sierra Leone's environment minister Jiwoh Abdulai.
"Enforcing the laws and policies is a major challenge," he added. He said that he was working on a new reforestation policy and that authorities regularly demolished the temporary structures that people were putting up in the national park.
Maada Kpenge, managing director of the state-owned Guma Valley Water Company that services Freetown, told Reuters that the consequence was already being felt in water shortages and that it was "serious."
Forest areas around cities in West Africa are constantly under threat as urban areas expand. Ivory Coast was forced to build a concrete fence to protect the park land near its sprawling commercial capital Abidjan.