The private helicopter hit several apartments near the top of the 30-storey building before plunging to the ground, said the National Emergency Management Agency.
Two pilots - the sole passengers of the aircraft owned by LG Electronics - were found dead. No residents of the building were killed or injured, an agency official told AFP.
"It appears that the pilots lost visual due to extremely foggy weather," said the official who declined to be named.
The helicopter was set to pick up an LG executive in Gangnam for a business trip to the southern city of Jeonju, Jeon Myeong-Wu, LG's
PR executive said in an interview with the YTN news channel.
TV news footage showed windows broken and walls collapsed at the luxury apartment block and the crumpled wreckage of the helicopter with the LG logo.
Hundreds of police officers, rescue workers and soldiers have been deployed to help clear the wreckage and evacuate residents after the rare accident.
The upscale district of the South's capital -- home to high-end shopping malls and million-dollar apartments -- was made famous by last year's global hit song "Gangnam Style" by South Korean rapper Psy.