Donald Trump has done it again, stumbling over another commonplace name in a speech during a campaign event.

This time, it was the southeast Asian country of Thailand that proved a problem for the US President, who pronounced it "Thigh-land".

"Shifting production to Thigh-land," Trump said during the speech at a washing machine factory in Ohio, before correcting himself.

"Thailand and Vietnam, two places that... I like their leaders very much."

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Twitter users didn't hold back in mocking Trump, leaping on the opportunity to post a series of gags and memes after video of the gaffe went viral.

The very public slip-up was the second this week for Trump, who earlier struggled to pronounce the United States' most famous National Park, labouring over the word Yosemite in a speech at the White House.

Trump was signing the Great American Outdoors Act, legislation that will devote nearly US$3 billion annually to conservation, when he mispronounced the name of the park.

Speaking about the need to preserve the American wilderness, Trump started strongly enough by telling the crowd: "We want every American child to have access to pristine outdoor spaces, where young Americans experience the breathtaking beauty of the Grand Canyon."

When he turned his attention to Yosemite National Park, the wheels fell off.

"When their eyes widen in amazement as Old Faithful bursts into the sky, when they gaze upon Yo-semites, Yo-seminites, towering sequoias, their love of country grows stronger and they know that every American has truly a duty to preserve this wondrous inheritance," Trump said.

Trump was widely mocked online for the gaffe, coming as public attention has been focused on the mental acuities of both presidential candidates.