Samsung Electronics has forecast a 60% fall in quarterly operating profit from a year ago, missing estimates, because of slowing Galaxy smartphone sales.
The world’s biggest mobile phones and TV maker said it expects an operating income of 4.1tn won ($3.8bn; £2.5bn) for the three months to September.
That is below analysts’ expectations for earnings of 5.2tn won.
The company’s full financial results will be released later this month.
Samsung’s mobile division, its biggest business, has been struggling to maintain its dominance against rivals such as Apple and Chinese smartphone-makers Xiaomi and Lenovo.
Its flagship Galaxy smartphone line has been losing market share to cheaper models that also have large screens and multiple features.
The South Korean company said quarterly sales amounted to 47tn won, which was below analyst estimates for 50.3tn won.
“Smartphone shipments increased marginally amid intense competition,” Samsung said in a statement.
“However, the operating margin declined due to increased marketing expenditure and lowered average selling price.”
The company also said it is “preparing new smartphone line-ups featuring new materials and innovative designs, as well as a series of new mid-to-low end smartphones”.
Samsung shares rose about 1.6% in Seoul despite the weak profit outlook.
Samsung announced on Monday that it would spend about $15bn on a new semiconductor plant in South Korea, to meet the growing demand for memory chips.
It is set to be the biggest single investment in a chip factory, and construction will begin in the first half of next year with operations due to start in 2017.
Samsung is a major chip supplier to other electronics firms, including major rival Apple.
Samsung is also facing some pressure at its consumer electronics division, which makes televisions, air conditioners and other appliances.
Sales and overall profit are forecast to have dropped because currency fluctuations made Japanese-made rival products cheaper to buy.
The Korean won strengthened about 3.5% against the Japanese yen in the third quarter.