Americans' personal income rose 0.9 percent last month while consumer spending jumped 1.4 percent, signaling to some economists a rebound in the economy's strength.

The increase in consumer spending had been on a downward trend since April and declined in real terms in July and August, the Commerce Department said. The increase in personal income was the largest since June, Commerce said.

The figures seemed to contradict some economists' fears that the recent slowdown in consumer spending suggested a recession sometime next year. The downward trend in consumer activity has contributed greatly to the slower growth in the nation's output from a 10.1 percent rate in the first quarter to a 3.6 percent rate in the third quarter.

Consumption constitutes about 60 percent of the nation's gross national product and personal income greatly affects consumer spending.

Economists yesterday said that despite the increase in consumption during September, the preliminary estimate of GNP for the third quarter due out this morning should show a decline from the 3.6 percent "flash" estimate made last month.

Growth in GNP probably will be revised downward because of the $6.1 billion decline in July and $6.4 billion drop in August in inflation-adjusted personal consumption.

"The increase in income was definitely a good one," said Allen Sinai, chief economist for Shearson Lehman/American Express. "But the spending increase comes off an extremely depressed summer. The one-month picture is definitely a rebound . . . and should arrest fears of a recession for the time being.

"I think consumer spending will speed up again," Sinai said. But he said there will be a "much slower pace of consumption and more saving."

Miriam Goldberg, an economist for Wharton Econometrics, said a more important indicator of future activity than personal income is a component of that measure -- wages and salaries -- which grew by $11.7 billion, the largest monthly gain since June.

The wages and salary figure "is shown to have a little more of an effect on consumption expenditures," Goldberg said. "Right now we're forecasting stable expenditures" for the rest of the year and "more of an increase in early 1985."

A pause in consumer spending during the summer was not unusual because it followed the satisfying of demand from the recession, Goldberg said.

Personal income increased $26.1 billion last month to an annual rate of $3.07 trillion, Commerce said. Personal outlays rose $34.6 billion to $2.46 trillion. In August, personal income rose $18.2 billion and personal outlays increased $200 million.

Disposable personal income -- personal income minus taxes -- rose 0.8 percent in September, following increases of 0.6 percent in August and 0.7 percent in July.

Personal consumption expenditures rose 1.4 percent in September after remaining flat for the previous two months.

Wages and salaries rose $11.7 billion in September, compared with $3.8 billion in August. Manufacturing payrolls declined $1.8 billion in September, following a $2.4 billion increase in August. About one-third of the decline in payrolls was accounted for by the United Auto Workers union strike against General Motors Corp.

Personal saving declined $12.7 billion last month, compared with increases of $15.7 billion in August and $14.6 billion in July.