For the first time in two years, the monthly suicide number topped 3,000 in the month of May, the National Police Agency said.
The report says suicides in Japan totaled 3,281 in May 2011, up nearly 20% from the same month last year.
The spike reverses a steady decline in the number of suicides since the end of last year, when the government launched a national public awareness campaign.
Japan was struck by a devastating earthquake and tsunami on March 11, and continues to grapple with a nuclear meltdown at the Fukushima Daiichi nuclear plant.
National suicide figures increased the two months following the disaster, the figures show.
The most populous region of Tokyo saw the highest number of suicides in May at 325, the agency said.
In the area hit hardest by the nuclear crisis, Fukushima saw 19 more suicides in May 2011 compared with May last year, with a total of 68.
The tsunami-devastated prefecture of Miyagi showed no change in its suicide numbers and Iwate prefecture saw a decline of three.
Northern Japan, which was devastated by the tsunami, had the highest rate of suicide in the country in 2010, according to the police agency.
Iwate prefecture in 2010 had the second-highest suicide rate.
Fukushima and Miyagi prefectures last year had a higher suicide rate than Tokyo.
Japan's suicide rate remains one of the highest among developed nations, according to the World Health Organization.