The unemployment rate jumped from 5.7 percent to 6.2 percent in July, more than erasing its encouraging June drop, the Labor Department reported yesterday.

The number of persons with jobs declined by 400,000 last month, the first time there has been a month-to-month fall in total employment since July 1977, and the largest one-month decline since January 1975.

The increase in the unemployment rate was caused almost entirely by the decline in jobholders. The number of persons joining the labor force grew only slightly.

Administration spokesmen such as Labor Secretary Ray Marshall had pointed to the June drop in joblessness from 6.1 percent to 5.7 percent as evidence of the success of the Carter administration's job-creation programs.

But yesterday, White House spokesman Rex Granum conceded that "the July figures indicate that the strong drop in June was a statistical aberration."

At 6.2 percent, the unemployment rate is hovering in the range that "has prevailed throughout most of the year," the Labor Department said yesterday.

The Labor Department said the number of unemployed Americans rose by 440,000 last month, with nearly all the increase occuring among teen-agers and adult women. It was those two groups that showed the greatest improvement in June.

Janet Norwood, acting commissioner of the Bureau of Labor Statistics, told Congress yesterday that it would be "premature" to look at the July unemployment increase and conclude that the economy is weakening, although she said it "is possible that subsequent data may reveal that July was the beginnig of a new trend."

Granum emphatically denied that the rise indicated any weakening. He said that other signs point to continuing economic growth. The administration expects growth to be in the 3.5 to 4 percent range for the rest of the year, about the same as in the first half.

That rate of growth should mean that any improvement in joblessness would come slowly.

Granum told reporters that the administration is still ahead of the unemployment goal it set in January. By the end of the year, according to revised projections released last month, the administration hopes to reduce the unemployment rate to 5.9 percent. After the June decline, some administration officials believed the government would surpass that goal.

As part of a new campaign to upgrade the Carter administration's public image, White House strategists have pushed hard to show how well the president has handled the unemployment problem and how little the public has understood its success in that area. Whether the July numbers will trigger a revision of that strategy is unclear.

Despite last month's climb, the unemployment rate remains nearly a full percentage point below last June's 7.1 percent rate, and about 3.5 million more Americans are at work today than a year ago.

Because of the steady improvement is joblessness over the last year, the president has shifted the focus of his economic policies from fighting unemployment to dealing with continuing acceleration in prices.

Based on a survey of 57,000 households, the Labor Department said the number of employed fell by 400,000 in July (some of the 440,000 unemployment rise came among those who just started looking for work), after rising 100,000 in June.

Another, less comprehensive survey of employer payrolls showed an increase in jobholders of 263,000 in July, following a 300,000 rise in June. Although the two surveys do not measure the same thing over the long run both tend to show the same gains in employment.

Norwood, testifying before the Joint Economic Committee, said that the discrepancy between the surveys in part can be attributed to problems in adjusting the labor force data to account for seasonal variations.

Statisticians usually have the most trouble with seasonal adjustments in June, when actual joblessness peaks as large numbers of students join the labor force, and again in September, when they go back to school. There are also problems in December and January, when employers hire for the Christmas rush, then let people go after the first of the year.

Last month, however, not only did the adjusted unemployment rate rise, the unadjusted unemployment rate also grew for the first time in years, analysts said.

The unemployment rate for all teen-agers rose from 14.2 percent to 16.3 percent, with almost all the increase coming among white teenagers. The black youth unemployment rate fell insignificantly from 37.1 percent to 37 percent.

Among adult women, the rate rose from 6.1 percent to 6.5 percent, while the rate for adult men increased to 4.1 percent form 3.9 percent.

The black unemployment rate increased to 12.3 percent from 11.9 percent.

The total number of persons in the labor force was 100.6 million in July, of whom 94.4 million had jobs and 6.2 million were unemployed and looking for work.