Samsung's "Gear Live" and LG's "G Watch" both powered by Android Wear are the first devices to adopt the new Google software specifically designed for wearables.
G Watch LG's first smartwatch is also equipped with Google's voice recognition service and can perform simple tasks including checking email, sending text messages and carrying out an online search at users' voice command.
The two devices cannot make phone calls by themselves but can be connected to many of the latest Android-based smartphones, the South Korean companies said in separate statements.
Samsung and LG are the world's top and fourth-largest smartphone maker, respectively.
A typical smartwatch allows users to make calls, receive texts and e-mails, take photos and access apps.
G Watch opened Thursday for online pre-order in 12 countries including the United States, France and Japan before hitting stores in 27 more including Brazil and Russia in early July.
Gear Live was also available for online pre-order Thursday.
The launches come as global handset and software makers step up efforts to diversify from the saturated smartphone sector to wearable devices.
Samsung introduced its Android-based Galaxy Gear smartwatch last year but it was given a lukewarm reception by consumers.
The second edition, Gear 2 (Review), was unveiled in February and based on Samsung's own Tizen softwear in a move to break free of its heavy reliance on Google's Android platform.
Samsung's arch rival Apple is believed to be set to launch its own smartwatch soon, while Google is moving towards a wide consumer launch of its eyewear Google Glass later this year.
Chinese smartphone maker Huawei also unveiled a connected watch called TalkBand.
Global smartwatch sales are expected to grow this year by more than 500 percent from 1.9 million units in 2013, according to the market researcher Strategic Analytics.
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