The company, known for catering to some of America's wealthiest consumers, said the effects of the weakening economy were evident even among its more established members with excellent credit.
For the period ended June 30, the company reported net income of $653 million, or 56 cents per share, compared with $1.06 billion, or 88 cents per share, in the year-ago period.
Analysts, on average, expected earnings of 83 cents per share, according to Thomson Financial.
Shares in American Express plunged $4.50, or 11 percent, to $36.40 in aftermarket trading. Its shares are down about 21 percent for the year.
The results include a $374 million addition to credit reserves, reflecting higher credit losses and the expectation for increased write-offs in the third and fourth quarter.
The company's U.S. card services division reported a profit of just $21 million, down from $580 million a year ago. Revenue net of interest expense in the segment rose a modest 1 percent to $3.6 billion. Results were hurt by a $1.5 billion provision for loan losses, up from $640 million in the 2007 quarter.
The net loan write-off rate, including both on-balance-sheet cardmember loans and off-balance-sheet securitized cardmember loans, was 5.3 percent, compared with 2.9 percent in the prior-year quarter.
Chairman and Chief Executive Kenneth I. Chenault said in a statement that fallout from the weakening economy accelerated in June with consumer confidence dropping, unemployment rates rising and home prices falling at "the fastest rate in decades."
"Consumer spending slowed during the latter part of the quarter and credit indicators deteriorated beyond our expectations," Chenault said. "The scope of the economic fallout was evident even among our longer term, superprime cardmembers."
In a conference call with analysts, American Express executives said the company has begun to notice problems even among cardholders with credit scores ranging from 650 to 750, and those who hold mortgages on multiple properties. As a general rule, those with a credit score above 650 receive the lowest interest rates.
The pinch felt by American Express' superprime cardholders mirrors a similar trend among borrowers at JPMorgan Chase & Co.
The bank said last week that even its more creditworthy borrowers are now failing to make their mortgage payments - the charge-off rate for prime mortgages nearly doubled from the first quarter to the second. CEO Jamie Dimon said his expectation is that those losses could triple during the remainder of 2008.
If economic conditions worsen, American Express said it will be forced to add to its credit reserves.
Quarterly revenue rose 8 percent to $7.48 billion, slightly below analysts' estimate of $7.6 billion. Revenue was boosted by strong growth in the company's international card services segment, as well as its global network and merchant services division and global commercial services unit.
Revenue from its international card services division increased 20 percent to $1.3 billion, due to higher cardmember spending and borrowing.
American Express no longer forecasts long-term earnings-per-share growth of between 4 percent and 6 percent, based on expectations of continued economic uncertainty and higher write-off levels. The company expects third- and fourth-quarter write-off rates to be higher than June levels.
The lender also hinted at various cost-trimming initiatives, including possible job cuts, that would result in restructuring charges in the second half of the year.
