SEOUL—Samsung Electronics Co.'s market value dropped by nearly $9 billion as investors remained concerned about slowing profit growth at the company's mobile division.

Investors are worried that the world's largest maker of mobile phones will continue to report slower profit and sales growth in smartphones as competition intensifies and less-expensive devices push down selling prices globally. Mobile devices have been the main profit driver for the South Korean company over the past two years, accounting for more than 50%...