Torrential rain since Saturday has triggered serious flooding across Central Java, with thousands of homes inundated, said disaster agency spokesman Sutopo Purwo Nugroho.
"Initial data... shows the impact of the floods and landslides in Central Java has caused 24 deaths, with 26 others still missing," Mr Sutopo said in a statement.
Footage aired by local broadcasters showed villagers sitting on their roofs to escape the rising water, their cars and homes submerged in brown water.
Areas prone to landslides in the densely-populated province were the worst hit, with drivers swept off roads and dozens of homes destroyed by fast-moving walls of mud, rock and water.
In one district, five people were buried in their homes as a landslide swept through the village. In another, nine people were killed as they tried to clear rubble from a road.
"Suddenly a huge landslide struck the cars and people on the street. Nine bodies were retrieved," Mr Sutopo said in a statement.
Search and rescue crews are combing villages for the 26 still missing, Mr Sutopo added.
Temporary shelters have been erected to help those evacuated from the disaster zone.
Landslides are not uncommon in Indonesia.
Last month, 15 students holidaying at a popular tourist spot in western Indonesia were killed when a landslide swept through their camp ground.