"I am quite concerned ... about recent moves by the Parliament to weaken the ability of prosecutors and law enforcement officials to combat a wide range of serious crimes, including bribery and corruption," said ambassador Nicholas Taubman.
Senior Romanian officials had briefed him on the "serious, negative impact" that the proposed changes would have, he said.
"The investigative tools and techniques that these parliamentarians want to eliminate in Romania are the very same ones that are widely used elsewhere in Europe and in the United States to protect the public good and to hold officials at all levels accountable.
"This is not just my view," added Taubman.
"A broad spectrum of Romanian and international experts have expressed concern that the proposed amendments, if enacted in law, would represent a real setback in Romania's efforts to fight many types of serious crime and corruption."
Taubman was speaking at a meeting of the Romania Civil Society Strengthening Program. His comments were posted on the US embassy website.
His remarks were supported by the British embassy to Romania, which said in a statement that it shared the concerns of the US government.
But Taubman's remarks drew a sharp response from the Speaker of parliament, Bogdan Olteanu.
Olteanu retorted that Taubman owed his diplomatic appointment to having funded the election campaign of US President George W. Bush, "which in Romania would be considered an act of corruption, an immoral act."
"If (Taubman) wants to hold forth on the limits to liberty set out in the laws adopted in the United States after 2001, he is welcome to do so," he added.
The proposed changes to the criminal code at the centre of the controversy have not yet been put into effect, as President Traian Basescu has asked parliament to look at them again.
Romania's political scene has been shaken by a series of corruption scandals in recent months.
On October 12, Decebal Traian Remes resigned as agriculture minister amid charges of corruption and EU warnings over Bucharest's handling of farm subsidies.
He was the third minister in the government of Prime Minister Calin Tariceanu to face legal charges for corruption.
On Wednesday, the European Commission gave Romania until December 16 to resolve outstanding concerns about its agricultural subsidies system, or face losing millions of euros in farm aid.