NEW YORK, Nov 22 (Reuters) - Upstart online retailers Temu.com and Shein are drawing millions of window-shoppers to their websites this holiday season, but they far lag market leader Amazon.com where it counts - turning those visits into actual sales.

Nine out of ten visitors to their websites were window shoppers, not buyers, according to Similarweb, a website data tracking firm.

Singapore-based Shein this year added kitchen mixers at $100, $10 bathroom holiday decorations and $6 headphones alongside clothing such as $13 dresses and $16 jeans. PDD Group's Temu sells $9 dresses and $12 women's shoes.

Shein's website drew 28.6 million unique monthly visitors in October, up 7.25% from a year earlier, according to Similarweb. But visits to Shein's website that resulted in actual transactions declined to 4.1% from 4.6% a year earlier.

Amazon trounces both online retailers with 56% of its 268 million unique monthly visits in October resulting in purchases, its data shows.

Temu, which launched in the U.S. in September 2022, is offering deals for cheap goods for the holiday shopping season. It had 42 million unique monthly visitors in October 2023, more than four times the amount a year earlier. Only 4.5% of those resulted in actual transactions.

The additional eyeballs on the retail websites is thanks to aggressive marketing. Shein is ramping up holiday marketing in the U.S. and Europe as it extends its reach and products on its platform. The company's valuation rose to $90 billion after it acquired U.K.-based online retailer Missguided in October.

In Europe, the Temu app overtook eBay in October, hitting 51 million monthly active users across Android and Apple devices, according to data.ai. The Shein app had 64 million monthly active users, a two-thirds increase on a year ago.

Shein increased its ad spend 60% in the first two weeks of November compared to the same period a year earlier, according to data from marketing intelligence firm SensorTower. Its rival, Temu, doubled its ad spend within the same time frame, the data showed. SensorTower does not provide dollar amounts for ad spend. In New York's Times Square, Shein in November launched a pop-up shop in the Forever 21 store, highlighting home goods and apparel along with its first Times Square billboard.

Shein has been able to trump fast-fashion incumbents Inditex's Zara and H&M, with revenues reaching $24 billion in the first three quarters of this year, according to reports.

But the fast-fashion giant continues to face more challenging competition from other online marketplaces such as Temu.com and Amazon.

Melissa Minkow, director of retail strategy at information technology firm CI&T, said that online shoppers have an "inherent comfort level" with Amazon's expansive product categories, shipping speeds and return policies compared to Shein and Temu.

At Amazon, which hosted a two-day Prime event in October, purchasing visits and unique visitors in the U.S. both increased in October compared to a year earlier, according to Similarweb.

Amazon cut its spending on advertising for the holidays in early November compared to a year ago, but saw more user engagement than it did in 2022, according to SensorTower.

The competitive landscape continues to heighten as shoppers get closer to Black Friday.

Peter Pernot-Day, head of corporate strategy at Shein, said Shein has advised its marketplace sellers "to optimize their product listings" by "providing competitive pricing and keeping inventory up to date."

(Reporting by Arriana McLymore in New York, Additional reporting by Helen Reid in London Editing by Nick Zieminski)