"Bitcoin at this time plays a very small role in the payment system. It is not a stable source of value and it does not constitute legal tender. It is a highly speculative asset," said Yellen, answering a question by CNBC's Steve Liesman.
Yellen has offered little commentary on the surging crytpocurrency during her tenure. She did say back in 2014 that the Fed did not have the authority to regulate bitcoin and reiterated that point on Wednesday.
"The Fed doesn't really play any regulatory role with respect to bitcoin, other than assuring that banking organizations that we do supervise are attentive, that they are appropriately managing any interactions they have with participants in that market, and appropriately monitoring anti-money laundering, bank secrecy act responsibilities that they have," Yellen said.
In response to a separate question, the Fed chair said some global central banks are looking at experimenting with their own digital currencies, but that isn't something the Fed would pursue in the near future.
"I really want to caution, this is not something the Federal Reserve is seriously considering at this stage," Yellen said. "While we're looking at research on this topic, there are, I think to my mind, limited benefits from introducing it, a limited need for it and some substantial concerns."
The Fed chief was speaking at a press conference following the central bank's decision to raise interest rates a quarter point. This is her final such press conference before she is likely to be replaced by President Trump's nominee Jerome Powell next year.
She also added that the risk to U.S. financial institutions was "limited" even if bitcoin were to drop in prices.
The cryptocurrency is up 2,000 percent in the last 12 months and was last at around $16,500 on the Coinbase exchange. Futures on bitcoin began trading for the first time on a major exchange Sunday, helping to fuel this year's rally as the emerging market tries to go for legitimacy.
--With reporting by Evelyn Cheng