More than 56 million Social Security beneficiaries will see their checks increase 1.7% starting in January 2013, under an annual cost-of-living adjustment that is tied to how much certain prices climb in July through September compared with a year earlier. Eight million people who receive Supplemental Security Income—mainly the poor and disabled—will get the boost starting in December, the agency said.

This means the average monthly Social Security check will rise by $21 to $1,261, the agency said. However, the increase may be partially or completely offset by increases in Medicare premiums—the portion of a retiree's check that the government deducts to cover health-care expenses. The premiums for 2013 haven't been announced yet.

Similar adjustments also will boost the pension checks of millions of retired federal civilian workers and retired members of the military.