Moody's set a stable outlook to the ratings for four of the groups but kept a negative outlook for ING Bank, meaning it could cut it again.
The downgrades will only add to pressure on European leaders to sort out the region's debt crisis, with a real test to the union coming this weekend as Greeks go to the polls. Moody's also warned that, were Greece to exit the euro, further ratings actions on European banks could well be needed.
The long-expected news had little immediate impact on financial markets in Asia, with the euro holding firm around $1.2616.
"Today's actions reflect Moody's view that Dutch banks will face difficult operating conditions throughout 2012 and possibly beyond," Moody's said in a statement.
The agency said there were heightened risks for creditors amidst elevated uncertainty and downside risks to the economic outlook and fragile investor confidence in Europe.
Moody's agency said it had cut the ratings by two notches to Aa2 for Rabobank Nederland, to A2 for ING, to A2 for ABN AMRO Bank, and to Baa2 for LeasePlan Corporation.
The long-term debt and deposit ratings for SNS Bans were downgraded by one notch to Baa2. The short-term ratings for all the groups were unchanged.
Moody's said it had factored into the ratings an increased risk of Greece leaving the euro area, but this was currently not the central scenario.
"If a Greek exit became Moody's central scenario, further rating actions on European banks could well be needed," it added.
Moody's said the negative outlook for ING took into account the bank's funding structure, which relies substantially on wholesale funds and a significant amount of non-domestic deposits.
Dutch bank and insurer ING received 10bn euros in state aid during the 2008 financial crisis.
It was subsequently forced to separate its banking and insurance businesses and sell off various assets to meet European Commission requirements for state aid. The disposal could also help to raise money to repay state aid.