Speaking to Fox Business during a rambling telephone interview, the president said: “If Japan is attacked, we will fight World War III. We will go in and protect them with our lives and with our treasure.
"But if we’re attacked, Japan doesn’t have to help us. They can watch it on a Sony television."
The statement is the latest in a series made by Mr Trump bemoaning existing military agreements with Japan.
Mr Trump was referring to a decades-old treaty that guarantees US aid to Japan if the east Asian nation is ever attacked.
A Bloomberg report suggested the president was considering withdrawing from the accord, which has been in place since the Second World War.
Mr Trump regards the treaty as one-sided, three sources were quoted by Bloomberg as saying.
However, he is yet to take any steps towards ending the agreement and such a move is highly unlikely, the report said, citing people familiar with the matter.
Mr Trump has also talked about seeking compensation for relocating the US base in Okinawa, Bloomberg said.
The two countries are engaged in trade negotiations, with the Trump administration seeking to lower America's trade deficit and boost exports to its major trading partners.
The president has previously said he is unhappy with Japan's trade surplus and wants a two-way agreement to address it.
On Tuesday he railed against other countries' apparent reliance on US military power in the Middle East, singling out Tokyo and Beijing.
"Why are we protecting the shipping lanes for other countries (many years) for zero compensation," he wrote.
The threat of war in the region has intensified since a US drone was shot down last week.
Asked about the prospect of a conflict in the same Fox interview, he said: "I hope we don't but we're in a very strong position if something should happen.
"I'm not talking boots on the ground ... I'm just saying if something would happen, it wouldn't last very long."
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