Major banks have masked their risk levels in the past five quarters by temporarily lowering their debt just before reporting it to the public, according to data from the Federal Reserve Bank of New York.

A group of 18 banks—which includes Goldman Sachs Group Inc., Morgan Stanley, J.P. Morgan Chase & Co., Bank of America Corp. and Citigroup Inc.—understated the debt levels used to fund securities trades by lowering them an average of 42% at the end of each of the past five quarterly periods, the data show. The banks, which...