"It was fair to say it was one of the factors that factored into today's decision," she said at a news conference after the Fed kept interest rates unchanged, as was widely expected.
Yellen also said Brexit "could have consequences in turn for the US economic outlook.
Amid worries about slowing job growth, Federal Reserve officials remain only tentatively committed to two more rate hikes this year, and provided indications Wednesday that there might be only one.
Market expectations for a Fed rate hike dwindled ahead of the meeting, amid a lackluster May jobs report and concerns about a British exit from the European Union.
Gold prices surged after the announcement, briefly hitting $1,300 an ounce, while yields traded lower, with the benchmark 10-year note yield near 1.6 percent.
According to the CME Group's FedWatch tool, expectations for a hike were just 2 percent ahead of the announcement.
Follow CNBC's coverage of the June Federal Reserve statement and press conference below: