The rate is the highest for August since 1992, when the local economy was recovering from the 1990-1991 recession. The rate has remained above 6 percent since 2009.
The increase comes at a time of lackluster job growth on the Island. Last week, the department reported the local economy had just 2,700 more jobs in August than a year earlier.
A department economist believes the rate jump also reflects a rising number of jobless people who have resumed their job search. The unemployment rate doesn't include people who have given up looking for work.
"People have decided to re-enter the labor force because of the strong job growth earlier in the year or were forced to re-enter due to economic circumstances," said Shital Patel, labor-market analyst in the department's Hicksville office.
The number of unemployed workers rose to 116,000 in August, up 12,700 from a year earlier. Meanwhile, the number of employed dropped to 1.36 million, down 10,600.
Martin Cantor, director of the Long Island Center for Socio-Economic Policy in Melville, believes the official local unemployment rate doesn't reflect the true level of misery because "the unemployed increased substantially."
The department uses year-over-year comparisons because local data aren't adjusted to account for seasonal fluctuations. So month-to-month comparisons can be misleading because of seasonal factors.
The Island's unemployment rate compares with 8.8 percent for the state and 8.2 percent for the nation, both unadjusted for seasonal factors.
Elmont resident Nadia Davis joined the ranks of the unemployed on Dec. 31, when her job at a Manhattan-based management consulting firm was outsourced to India. To improve her chances of landing a job, Davis, who is in her 30s, graduated from Stony Brook University's Corporate Education and Training Program two weeks ago.
She said too many prospective employers treat the unemployed like pariahs: "Once you are unemployed, no one wants to take a chance on you."