Daniel Radcliffe hopes JK Rowling's comments about gender will not "taint" the Harry Potter series for fans.
In a statement posted on an LGBT suicide prevention charity website, the actor said: "Transgender women are women.
"Any statement to the contrary erases the identity and dignity of transgender people."
JK Rowling had been criticised for tweets taking issue with the phrase "people who menstruate".
Radcliffe said this was not about "in-fighting" and added he felt "compelled to say something" because Rowling was responsible for the "course his life has taken".
'Love is the strongest force'
Writing on The Trevor Project's website he said he was sorry to anyone whose "experience of the [Harry Potter] books has been tarnished".
"If these books taught you that love is the strongest force in the universe... that dogmatic ideas of pureness lead to the oppression of vulnerable groups; if you believe that a particular character is trans, non-binary, 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.
"I hope that these comments will not taint that too much."
The actor started supporting the charity, which provides suicide-prevention counselling to young people in the US, in 2009.
Rowling had tweeted at the weekend about an article discussing "people who menstruate".
"I'm sure there used to be a word for those people. Someone help me out. Wumben? Wimpund? Woomud?"
In response, she was called transphobic.
Rowling stood by her comments, saying it "isn't hate to speak the truth".