Eddie Redmayne is the second major Wizarding World star to speak out against its creator JK Rowling and in support of trans people.
"I disagree with Jo's comments. Trans women are women, trans men are men and non-binary identities are valid," the actor said in a statement.
Harry Potter actor Daniel Radcliffe released a similar statement on Monday.
JK Rowling has been criticised for tweets taking issue with the phrase "people who menstruate".
Eddie Redmayne plays Newt Scamander in the Harry Potter spin-off film series Fantastic Beasts, which are written by JK Rowling.
He was also nominated for an Oscar in 2016 for his portrayal of a fictional version of trans woman Lili Elbe in The Danish Girl.
His statement, given to Variety, went on to say: "I would never want to speak on behalf of the community but I do know that my dear transgender friends and colleagues are tired of this constant questioning of their identities, which all too often results in violence and abuse.
"They simply want to live their lives peacefully, and it's time to let them do so."
What did Daniel Radcliffe say?
Daniel Radcliffe posted on the website of The Trevor Project, an LGBT suicide prevention charity.
"Transgender women are women," he wrote.
"Any statement to the contrary erases the identity and dignity of transgender people."
He added that he hopes JK Rowling's comments about gender will not "taint" the Harry Potter series for fans.
Radcliffe said this was not about "in-fighting" - instead he felt "compelled to say something" because Rowling was responsible for the "course his life has taken".
'Love is the strongest force'
"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.
What did JK Rowling say?
The author 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".
If sex isn’t real, there’s no same-sex attraction. If sex isn’t real, the lived reality of women globally is erased. 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.— J.K. Rowling (@jk_rowling) June 6, 2020
"My life has been shaped by being female. I do not believe it's hateful to say so.
"I know and love trans people, but erasing the concept of sex removes the ability of many to meaningfully discuss their lives."
'Identity and dignity'
Radcliffe said it's important not to "invalidate" transgender people's identities and "cause further harm".
"Transgender women are women. 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."
The actress who played character Cho Chang in the Harry Potter films, Katie Leung, also shared support for trans people.
So, you want my thoughts on Cho Chang? Okay, here goes...(thread)— Katie Leung (@Kt_Leung) June 7, 2020
Conversation online had touched on the representation of black, Asian and minority ethnic characters in the Harry Potter series, with Cho Chang trending on Twitter.
Leung tweeted: "So, you want my thoughts on Cho Chang? Okay, here goes..."
Instead of commenting, she used the rest of the thread to share links to charities helping black trans people.
Why is this such a fiercely debated topic?
Transgender people say they just want equal rights, but some groups believe that will lessen women's rights.
A key differentiation is between the words "sex" and "gender". Our sex, which is physical - male or female - is distinct from our gender, which is psychological and social.
"Women are oppressed on the basis of their biological sex, not their gender identity. There has to be a place for the female sex as a distinct group," said Stephanie Davies-Arai, who founded the Transgender Trend website - a place for parents to discuss trans issues.
This argument - the distinction between sex and gender - is refuted by some, including trans activist Julia Serano, who argues there are more than two discrete mutually exclusive sexes.
Instead, she argues sex is made up of a number of variable dimorphic traits - like chromosomes and reproductive organs - that sometimes align in a person and sometimes don't.
'A slap in the face'
JK Rowling's comments feel like "the biggest slap in the face", says Scarlet Marie, a Harry Potter fan who recently came out as a trans woman.
"Harry had all of this power and a world awaiting him, but he hid away. I related to that," she tells Radio 1 Newsbeat.
Scarlet was bullied at school and says the Harry Potter series "saved" her.
"No matter what torture I was feeling, I could escape it. What people said to me couldn't hurt me. I was no longer in Birmingham, I was at Hogwarts."
Now she no longer wants to read the books.
"As children, watching the movies, you'd with siblings and say 'I'll be Hermoine! I'll be Harry!'. She's taken that away. I know she thinks I can't be a woman because I wasn't born one.
"It feels like this magical world she created for me, where anyone can be themselves, wasn't real."
Rowling was criticised in December last year for defending a woman who lost her job after saying children cannot change their biological sex.
At Maya Forstater's employment tribunal, which she lost, the judge spoke about the "enormous pain that can be caused by misgendering".
JK Rowling used the hashtag #IStandWithMaya, and said women shouldn't be "forced" out of jobs for "stating that sex is real".