The landslide in Selangor state, on the outskirts of capital, Kuala Lumpur, occurred about 3 am (1900 GMT), tearing down a hillside into an organic farm with camping facilities, the state fire and rescue department said in a statement.
More than 90 people were caught in the landslide and 59 had been found safe, with 22 still missing, according to messages on social media by the department.
In addition to the 12 dead, eight were hospitalised, it said.
One of those taken to the hospital was pregnant, while others had injuries ranging from minor cuts to a suspected spinal injury, health minister Zaliha Mustafa told a news conference.
District police chief Suffian Abdullah said the dead were all Malaysians and included a child about 5 years old.
Almost 400 people from several agencies had been deployed, with search-and-rescue efforts ongoing, he told a news conference.
The landslide came down from an estimated height of 30 metres (100 ft) above the campsite, and covered an area of about one acre (0.4 hectare), according to the fire and rescue department's state director.
Footage from local television showed the aftermath of a large landslide through a steep, forested area beside a road, while other images on social media showed rescue workers clambering over thick mud, large trees and other debris.
"I pray that the missing victims can be found safely soon," Malaysia's minister of natural resources, environment and climate change, Nik Nazmi Nik Ahmad, tweeted on Friday morning, one of several ministers who were heading to the scene. "The rescue team has been working since early. I'm going down there today."
The disaster struck about 50km (30 miles) north of Kuala Lumpur in Batang Kali town, just outside the popular hilltop area of Genting Highlands, an area known for its resorts, waterfalls and natural beauty.
Pictures posted on the Father's Organic Farm Facebook page show a farmhouse in a small valley, with a large area where tents can be set up.
Selangor is the country's most affluent state and has suffered landslides before, often attributed to forest and land clearance.
The region is in its rainy season but no heavy rain or earthquakes were recorded overnight.
A year ago, about 21,000 people were displaced by flooding from torrential rain in seven states across the country.
