Exports to the US, the world's largest economy and the number one market for Japanese exports, fell 45.4 per cent in May, marking the 21st straight monthly decline, the finance ministry said.
In the same month US-bound auto exports plunged 54.8 per cent, while shipments of Japanese-made car parts to the US fell 47.8 per cent.
Both statistics make grim reading for Japanese automakers like Toyota and the raft of autoparts suppliers that depend heavily on the American market.
Japan's exports to the European Union, another key market, decreased 45.4 per cent.
Also in May Japan's trade surplus spiked to $3.1bn, the biggest in a year, but that was due to a sharp fall in imports further underscoring the weaknesses in the economy.
"Overall exports remained depressed, which squarely reflected the ongoing economic downturn," said Yu Ooki, a finance ministry official.
Japan's economy shrank at a 14.2 per cent annual pace in the first quarter of 2009 - better than many had predicted but still its worst quarterly contraction.
Last week Japan's economy minister, Kaoru Yosano, said the slump appeared to have bottomed out but warned a recovery will depend on the world economy.