TikTok was launched internationally in 2017. ByteDance introduced the overseas version of its Chinese short-video service in September that year.
The Chinese service, Douyin, had launched earlier, on September 20, 2016. Although TikTok and Douyin share similar technology and branding, they operate as separate services, with different markets and regulatory environments. That distinction is the source of many conflicting answers to this question.
ByteDance then accelerated TikTok’s international growth by acquiring Musical.ly in November 2017 for a reported price of up to $1 billion. In August 2018, Musical.ly was merged into TikTok, and existing users and content were moved to the TikTok platform. This merger is sometimes incorrectly described as TikTok’s original launch.
So 2016 refers to Douyin’s Chinese debut, 2017 to TikTok’s international launch, and 2018 to the major Musical.ly integration that helped make TikTok globally recognizable.