Ferns said later Monday in a statement officially announcing the switch that his attempts at independent thinking "have been met with threats and retaliation by leadership within the Democratic Party."
The GOP says Ferns' switch came one day before a deadline for candidates to switch parties on voter registration forms to be legally qualified to run for legislative office.
Miley said in a statement he was disappointed and not totally surprised by Ferns' announcement but wished him well.
During a meeting a few weeks ago with Miley and other House Democrats, Ferns "stated he would never become a Republican for reasons that I should not repeat publicly," Miley said. "... In politics, some people say one thing and do another. Delegate Ferns has now done this on multiple occasions."
A message left with Fitzsimmons wasn't immediately returned.
In July, Sen. Evan Jenkins, of Cabell County, switched to the Republican Party and announced his intentions to run in West Virginia's 3rd Congressional District race next year. The incumbent is Democrat Nick Rahall.
Ferns is a physical therapist and owns a therapy and fitness center in Benwood. He holds degrees in business marketing and physical therapy from Wheeling Jesuit University.
"We proudly welcome Delegate Ryan Ferns to our Republican family today," Republican state Chairman Conrad Lucas said in a statement. "Delegate Ferns had the courage to stand up for the Ohio Valley in Charleston, and the independence to take tough stands, regardless of party or issue."
In April 2012, Ferns pleaded guilty to a charge of drunken driving after Wheeling police said his blood-alcohol level was 0.229 percent, above the state's legal limit of 0.08 percent. He remained on the ballot, citing an outpouring of support, and was elected to a second term last fall.