Fire official Cha Yang-oh told reporters that a helicopter crashed in the upscale Gangnam district in southeastern Seoul.
The helicopter belonging to the LG Electronics was on its way to a nearby landing field where it was scheduled to pick up LG employees and head to a Korean city in south.
Cha said 57-year-old captain Park In-kyu died. A 36-year-old vice-captain survived the crash, but died later in a hospital. LG Electronics Inc. said the two were company employees.
Cha said no one was hurt in the apartment tower. The I-Park building is one of the most expensive apartment buildings in Seoul.
"I heard a blast sound while I was sleeping, so I ran out. The boom sounded like a war had started," said Jeong Young-woo, 54, an apartment tower resident.
TV footage showed shattered windows on the upper levels of the building. The helicopter was nearly destroyed save for its tail section.
Seoul was foggy on Saturday morning but Cha declined to say if the thick fog played a role in the crash.
Gangnam is Seoul's trendy district that became famous due to South Korean rapper PSY's smash hit music video "Gangnam Style."