Jalina Porter made the statement at a press conference on Friday after the Polish lower house of parliament in a surprisingly swift move passed a bill that was rejected by the Senate a few months ago.
The new regulations, which require a non-European Economic Area media owners to sell their majority stakes in Polish media companies and caps their ownership level at 49 percent, will affect only TVN, which is owned by US media giant Discovery.
TVN has been critical of the Polish government and has exposed a number of scandals related to government officials.
Porter said that "the United States is troubled by the passage in Poland today of a law that would gravely weaken media freedom in that country."
"We also encourage (Polish) President (Andrzej) Duda to reaffirm his past statements in support of freedom of expression, the sanctity of contracts, and the shared values that underpin our relationship," she added.
"We also strongly encourage him to act on these values in regards to this legislation that, if it does become law in its current form, could severely impact media freedom as well as the foreign investment climate," she said.
The final decision on the draft legislation is now in the hands of the Polish president, who can sign the bill into law, veto it or send it to the country's Constitutional Tribunal, which the opposition says has been politicised by the ruling party, to verify the legislation's constitutionality.
Duda previously said that he would protect media freedom and bilateral commitments, but his comment on Friday was not unequivocal.
"Of course, we will analyse it (the law) and an appropriate decision will be made," Duda said. "I've already spoken about the angle from which I will assess the bill, and I will make such a decision."
PAP has learned that US Republican and Democrat congressmen are also working on an appeal to Poland on the matter.
In the past, deputies from both houses of the US Congress sent letters to Poland criticising the new draft solutions.
Donald Tusk, leader of Poland's main opposition party, the centrist Civic Platform, told TVN24, a news outlet owned by TVN, on Friday that "it's not a dark day for Polish democracy, it's dark six years for Polish democracy," referring to the six-year-long rule of the conservative Law and Justice (PiS) party.
He said it was obvious for him that "for (PiS) Chairman (Jaroslaw) Kaczynski free media are the same problem that free courts, opposition and the constitution used to be, or still are".
"One can say that he's quite consistent in these attacks on the foundations of democracy," Tusk added. (PAP)