The UK prime minister met Chinese President Xi Jinping on Thursday as he made the case for a "more sophisticated" relationship between the two countries.
Asked by reporters early on Friday about the UK's efforts to strengthen economic ties with China, Trump expressed disapproval.
"Well, it's very dangerous for them to do that, and it's even more dangerous, I think, for Canada to get into business with China," he said.
He added that Canada was doing "poorly" and said "you can't look at China as the answer".
Canadian Prime Minister Mark Carney visited China earlier in January as the two countries sought closer economic ties, reaching a preliminary agreement to cut tariffs on selected goods.
The US president's comments come as Starmer concludes his visit to China.
He will meet businesses in Beijing on Friday as he continues his push to bolster economic ties with China on the final day of his visit to the country.
The prime minister will later travel to Shanghai before leaving for Tokyo to meet his Japanese counterpart Sanae Takaichi for a working dinner.
UK officials said the US was aware of Starmer's trip and his objectives in advanced, and pointed to Trump's visit to China expected to take place in April.
Starmer's visit to China's biggest city and global financial hub will cap a controversial trip during which the prime minister's political opponents accused him of "kowtowing" to Xi.
In a post on X, the Republican-led US House Foreign Affairs committee said: "To all world leaders meeting with Xi Jinping: China sells nothing but cheap products and cheap friendships."
Under deals agreed on the visit, tariffs on Scotch whisky are set to be halved to five per cent and UK citizens visiting China for fewer than 30 days will no longer need a visa.
Starmer stressed the financial benefits of an improved relationship with China, which is the world's second-largest economy and a major part of global supply chains, in his meeting with Xi.
He also addressed the UK-China Business Council, where chief executive of British pharmaceutical firm AstraZeneca Pascal Soriot announced an investment of 100 billion yuan ($A20.4 billion) in China by 2030 to expand medicines manufacturing and research and development.
Downing Street also left the door open to a future visit to the UK by Xi, whose last state visit to Britain was in October 2015.
with Reuters
