In a meeting Wednesday morning with U.S. retail industry leaders, Trump said, "H&R Block probably won't be too happy. That's one business that might not be happy with what we're doing."
Shares of H&R Block fell to session lows, down 2.6 percent on the day, before ending the day just 0.8 percent lower.
Trump has promised to unveil his own "phenomenal" tax plan within weeks after investors appeared to be worried that reform wouldn't happen until 2018.
With Wednesday's losses, the stock is down more than 9 percent year-to-date.