Gascon made the announcement at a news conference at City Hall with Mayor Gavin Newsom and District Attorney Kamala Harris. The three stressed that cooperation between city agencies and federal partners was the key to the decline.
"Good people are working together, and they're making a difference," Gascon said.
He has pledged repeatedly to try to make San Francisco the safest big city in the country.
"Although we're not there yet, we're certainly moving in the right direction," Gascon said.
From January through September, 5,361 violent crimes were committed throughout the city, down from 5,611 in the first three quarters of 2009 and 6,184 for the same period in 2008, according to the Police Department.
Property crimes decreased from 27,731 in the first three quarters of 2008 to 26,649 in the same period of 2009. There were 24,156 this year, the Police Department said.
Homicides have remained about the same, down from 38 in 2009 to 37 this year.
Gascon has made the tabulation of crime statistics in each neighborhood, with a concurrent response to new crime trends, a centerpiece of his administration.
He stressed, however, "There are lives. There are real people behind the numbers."
Newsom agreed there had been "very positive gains" in the fight against crime, even with "substantially" fewer resources.
Recent budget cutbacks have reduced police overtime and forced the halting of new police academy classes.