The economy added 169,000 jobs, the department's Bureau of Labor Statistics said. That is close to the average for the year, but short of the 180,000 jobs economists expected.
The last time the unemployment rate was at 7.3 percent was in December 2008.
Analysts say it takes monthly job growth of close to 300,000 jobs to lower the unemployment rate. The drop from 7.4 percent in July is due in part to a smaller labor force, as many people have given up looking for work, which takes them out of the statistical pool used to calculate the unemployment rate.
The department also lowered its estimate of jobs gained in June and July.
For June, the department said, 172,000 jobs were added to the economy, rather than the previous estimate of 188,000. For July, the 162,000 jobs created in the month was revised sharply lower to 104,000.
The unemployment rate has dropped from 8.1 percent in August 2012, although economists point out that much of the decline represents the failure to find work.
The department said in August there were 4.3 million persons unemployed for 27 months or more, which amounted to 37.9 percent of the 11.3 million persons counted as unemployed.
An additional 7.9 million are counted as working part time for economic reasons, which means working part time although they would prefer a full time job.
Another 2.3 million are counted as marginally attached to the workforce. This group, consisting of people who had looked for work in the past year, but not in the past month, is not counted as part of the workforce, although they would push the number of unemployed from 11.3 million to 13.6 million.