Radcliffe, who played the title character in the films based on Rowling's Harry Potter book series, apologized to fans Monday after Rowling appeared to make anti-trans remarks on Twitter.
In an open letter on The Trevor Project website, Radcliffe voiced his support for transgender and nonbinary people.
"Transgender women are women," he wrote. "Any statement to the contrary erases the identity and dignity of transgender people and goes against all advice given by professional health care associations who have far more expertise on this subject matter than either Jo or I."
Radcliffe apologized to those who feel their experience with the Harry Potter books is tarnished or diminished by Rowling's remarks.
"I am deeply sorry for the pain these comments have caused you. I really hope that you don't entirely lose what was valuable in these stories to you," the actor wrote.
"If these books taught you that love is the strongest force in the universe, capable of overcoming anything; if they taught you that strength is found in diversity, and that dogmatic ideas of pureness lead to the oppression of vulnerable groups; if you believe that a particular character is trans, nonbinary, or gender fluid, or that they are gay or bisexual; if you found anything in these stories that resonated with you and helped you at any time in your life -- then that is between you and the book that you read, and it is sacred," he said.
On Saturday, Rowling was accused of being anti-trans after she took issue with the wording of a headline for an article, "Opinion: Creating a more equal post-COVID-19 world for people who menstruate."
"'People who menstruate.' I'm sure there used to be a word for those people," she tweeted in response. "Someone help me out. Wumben? Wimpund? Woomud?"
Rowling defended her remarks in a series of subsequent posts.
"I know and love trans people, but erasing the concept of sex removes the ability of many to meaningfully discuss their lives. It isn't hate to speak the truth," the author said.
"The idea that women like me, who've been empathetic to trans people for decades, feeling kinship because they're vulnerable in the same way as women -- ie, to male violence -- 'hate' trans people because they think sex is real and has lived consequences -- is a nonsense," she added.
Rowling was previously criticized for supporting a researcher who was fired due to transphobic tweets.