* GM saw 5th straight quarterly sales decline in China
* Mass-market brands sales all decline
* Luxury Cadillac sales continue to grow
* China's overall car market set to decline again in 2019 (Adds backgrounds, details from the company)
BEIJING, Oct 10 (Reuters) - General Motors Co's July to September vehicle sales in China fell 17.5%, as the U.S. automaker was hurt by a slowing economy amid the Sino-U.S. trade war and by heightened competition in its key mid-priced SUV segment.
GM delivered 689,531 vehicles in China in the third quarter this year, according to a company statement. The drop for the quarter ended September 30 marks the fifth straight quarterly sales decline for GM in China, the world's biggest auto market.
It delivered 2.26 million vehicles in the first nine months this year, according to Reuters calculation.
As GM and Ford Motor Co's China sales extend declines, U.S. car companies' share of total China passenger vehicles sales fell to 9.5% in the first eight months of this year from 10.7% in the year-ago period, according to the China Association of Automobile Manufacturers (CAAM).
Over the same period, German car makers' share has risen to 23.8% from 21.6% and Japanese auto makers' to 21.7% from 18.3%.
In China, GM has a joint venture with SAIC Motor Corp , in which the Buick, Chevrolet and Cadillac are made. It also has another venture, with SAIC and Guangxi Automobile Group, in which they make no-frills minivans and have started to make higher-end cars.
GM, the second biggest international automaker in China by sales, sold 3.64 million units in China last year, down from 4.04 units in 2017.
Sales of GM's affordable brand Baojun dropped 34.9% for the latest quarter, while sales of the mass-market Buick fell 20.6%. But luxury brand Cadillac's sales jumped 10.9%.
Annual industry car sales in China fell last year for the first time since the 1990s, and they are expected to fall this year too. Sales dropped 6.9% in August from the same month a year prior, CAAM said.
An official at the association said last month that in the next three years the industry could see "low or small negative growth".
CAAM is set to announce September sales next week. (Reporting by Yilei Sun and John Ruwitch; Editing by Muralikumar Anantharaman)