Parsley, rapeseed, mustard and perilla were found to be affected, although the levels were far from life-threatening.
It comes as more countries join Singapore in restricting food imports from the country over fears of contamination from the critical Fukushima Daiichi nuclear plant.
The US, Australia, Hong Kong, Canada and Russia have all taken measures.
The contaminated vegetables discovered in Singapore had come from the regions of Tochigi and Ibaraki - inside the affected area - but also more worryingly from two other areas, Chiba and Ehime.
The radiation levels mean that an adult would have to eat 3.5kg to get the same exposure as from a single x-ray.
Singapore has already stopped imports of milk, meat and related products from the affected area of Japan.
The US, Australia and Russia have also banned certain foodstuffs from around the damaged nuclear plant.
Hong Kong is requiring the Japanese authorities to screen exports, while Canada said it will carry out its own inspections.
Both Australia and Singapore have banned agricultural food imports from the regions of Fukushima, Gunma, Ibaraki and Tochigi.
Hong Kong included Chiba on its suspension list - the origin of one of the contaminated samples since discovered in Singapore.
Japan's food producers are under pressure after a deadly earthquake caused radiation to leak from the power plant, contaminating produce.
Australia's regulator, the Food Standards Australia New Zealand, said it was taking a precautionary measure that was consistent with approaches internationally.
Three UN agencies have issued a joint statement saying Japan had been taking the right actions.
"Food monitoring is being implemented, measurements of radioactivity in food are taking place and the results are being communicated publicly," said the International Atomic Energy Agency, the World Health Organization and the Food and Agriculture Organization.
The bans on food imports from Japan into other countries come after shipments of certain food products from the Fukushima area were halted domestically as well.
The government has told people living in Fukushima not to eat 11 types of green leafy vegetable that are grown locally, amid fears of radiation contamination.
Producers in the region have been asked not to send their goods to the market.
In the Ibaraki prefecture, all shipments of milk and parsley have also been stopped.
"Unfortunately, as the situation is expected to last for the long term, we are asking that shipments stop at an early stage," said Japanese Cabinet Secretary Yukio Edano.
"It is desirable to avoid intake of the foods as much as possible."
Analysts say radiation fears are likely to hit domestic demand, as most agricultural produce from Japan is consumed within the country.
Nonetheless, the effects of the earthquake and tsunami have thrown a shadow over Japan's exports.
Analysts expect a negative impact on export growth in the months to come.
The latest figures showed that exports had actually picked up in February.
Exports rose 9% in February from the same month last year, according to the Ministry of Finance.
However, those figures do not reflect the impact from the earthquake, which struck on 11 March.
The expected slump in food exports on top of that in manufacturing could start to weigh on those trade numbers.
Damage from the earthquake has caused major exporters like Toyota and Honda to suspend production.

The Leyton Orient defender, who used to play for Liverpool, was held in Brompton Road, Knightsbridge, on Tuesday, the BBC understands.
The Met Police said a 33-year-old man and a 31-year-old woman were arrested and taken to a west London police station.
They have been bailed until late April.
In a statement, Leyton Orient Football Club said: "The club are aware of the incident reported earlier today involving Andrea Dossena."
It added it would not comment further whilst it was being dealt with by the authorities.
Dossena scored against Manchester United and Real Madrid while he was at Liverpool.
After joining Liverpool for £7m in July 2008, Dossena scored in the Reds' 4-0 win against Real Madrid in the Champions League in March 2009 and then again in the club's 4-1 away victory at Old Trafford against Manchester United.
The Italian, who won 10 caps, joined Napoli in January 2010 before a spell at Sunderland in 2013-14.
He joined League One Leyton Orient in November last year.
Singapore has reported finding low levels of radioactivity in four vegetable samples imported from Japan.
[NEXT_CONCEPT]
Ex-Premier League footballer Andrea Dossena has been arrested on suspicion of shoplifting at London department store Harrods, the BBC has learned.