The United States leapfrogged Brazil and Mexico last year to become the world's largest debtor nation, with foreign interests owning $107.4 billion more in the United States than Americans own overseas, the government reported yesterday.

The deficit resulted from a $111.8 billion swing from the year before, when the United States registered a $4.4 billion surplus in its financial position with the rest of the world. As recently as 1982, moreover, the United States was the world's largest creditor nation.

"It's amazing to go from a small net creditor to the biggest debtor in one year," said C. Fred Bergsten, director of the Institute for International Economics and a Treasury Department official in the Carter administration.

He cited the "breathtaking pace of deterioration" as a major economic problem for the United States. "It shows how enormous the momentum is in the wrong direction" and how hard it will be to turn around, he said.

The debt is believed to have grown by $30 billion in the first three months of this year, and Bergsten predicted it will hit $500 billion before it levels off. Further, if the value of the dollar fails to continue falling, he said the debt could reach $1 trillion in the 1990s.

"That's levying a pretty big cost to the economy. That's the price of neglecting the problem for five years," he said.

A strong stock market surge late last year accelerated America's foreign debt, as the value of stocks owned by overseas investors gained in value. As recently as last Tuesday, when the Commerce Department released the broadest measure of U.S. international transactions, economists predicted that this country had ended 1985 as the No. 3 debtor nation, behind Brazil and Mexico.

Brazil, which until last year had been the world's leading debtor nation, owes $100 billion. Mexico owes $95 billion.

President Reagan has brushed off the significance of growing foreign debt as a problem for the economy, asserting that it shows the strength of America because so many foreigners want to invest their money here.

But many economists disagree. They say the large and growing debtor status of the United States casts a shadow over its economic viability, even though America is not in the position of Mexico, Brazil and other Third World nations of being unable to pay its large international debts.

"It is unsustainable and undesirable," said Bergsten.

"It's a very unhealthy development," agreed Larry Chimerine, chairman and chief economist of Chase Econometrics.

He said the debt drains income from the country because the United States has to pay interest and dividends on overseas investments in this country. "Fundamentally, what we are doing is importing goods and exporting bonds, or IOUs, if you will," said Chimerine.

The problem could worsen if the overseas investors decided to sell their holdings and the United States had to repay capital, as Mexico and Brazil are being asked to do in paying off their loans, instead of meeting only interest and dividend requirements.

Chimerine added that there won't be a turnaround until the United States curbs its merchandise trade deficts, which have set records for four straight years and reached $158.5 billion in 1985.

This is the first time the United States has been a debtor nation since 1914, when it owed foreign interests $2.2 billion more than they owed this country.