Public health officials said the sample did not affect the water supply but said further testing was being done as a precaution.
"The drinking water supply in Massachusetts is unaffected by this short-term, slight elevation in radiation," Public Health Commissioner John Auerbach said on Sunday, according to the Boston Herald. "However, we will carefully monitor the drinking water as we exercise an abundance of caution."
The rainwater showed low levels of radioiodine, known as I-131. Officials did not identify the location where the sample was taken but said the air in the area did not show increased levels of radiation.
The sample's concentration of I-131 was 79 pCi/L (picocuries per liter), The Boston Globe said. Even if people did drink the rainwater, it is "still 25 times less risky than it would need to be in order to cause any kind of health concerns" Auerbach said, according to the Globe.
"In Massachusetts, none of the cities and towns rely on rainwater as their primary source of water," Auerbach said, according to the Globe. "That's why we're so comfortable in saying that the drinking-water supplies throughout the state are pretty safe."
The sample from last week was among the more than 100 taken from around the nation as part of the Environmental Protection Agency monitoring system that checks for radiation.
Two primary reservoirs that supply water for 2.5 million people in Massachusetts, including Boston, are safe, officials said, according to WHDH-TV. In addition, 12 other water sources around the state were tested on Sunday, with results expected later in the week, according to reports.
I-131 has a half-life of eight days, Auerbach said, meaning that only half of the radiation will exist in eight days.
Officials said comparable amounts of radioiodine were found in rainwater in Pennsylvania, California and Washington. The fallout is believed to be linked to the crippled nuclear plant in Japan, which has leaked some radiation.
One expert said radiation from the Japanese plant is not dangerous to the continental U.S.
"The concentrations are so low as to be absurd," Ronald Ballinger, a professor of nuclear science and engineering at Massachusetts Institute of Technology, told the Boston Herald. "The event is pretty much contained, right now. They have power back to the site."