Thailand's King Maha Vajiralongkorn is due to arrive in China on Thursday, the first ever visit by a reigning Thai monarch.
The visit is at the invitation of Chinese President Xi Jinping to celebrate the 50th anniversary of both countries establishing diplomatic ties in 1975. China is believed to have been pushing for this visit for some years.
Thailand is still officially a military ally of the US, but relations with China have grown steadily closer in recent years.
In contrast, those with Washington have been frayed by US criticism of human rights in Thailand, by US President Donald Trump's tariffs, and a perception that the US is no longer as committed as it once was to its Asian friends.
In a statement, the Thai government said the trip "underlines the deep-rooted friendship and mutual understanding shared between Thailand and China at all levels".
