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.
The launches come as global handset and software makers step up efforts to diversify from the saturated smartphone sector to wearable devices.
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.