The Edinburgh-based author tweeted her support of Maya Forstater after the researcher lost an employment tribunal. Think tank Centre for Global Development's decided not to extend Forstater's contract earlier this year due to a dispute over her views on social media.
Forstater's publicised views were described as "offensive and exclusionary" and not in keeping with government proposals to allow people to self-identify as the opposite sex.
What is a TERF and what is transphobia?
JK Rowling tweeted: "Dress however you please. Call yourself whatever you like. Sleep with any consenting adult who’ll have you. Live your best life in peace and security. But force women out of their jobs for stating that sex is real? #IStandWithMaya #ThisIsNotADrill".
Three hours since Rowling's support was posted, the Harry Potter author's tweet has received 7.3k replies many of which have accused her "transphobia".
According to LGBT equality charity, transphobia is: "the fear or dislike of someone based on the fact they are trans, including the denial/refusal to accept their gender identity."
"JK Rowling is a TERF" also trended alongside the author's name. TERF is an acronym which stands for "trans-exclusionary radical feminist", or someone who doesn't view transgender women as women. The term was coined by Australian journalist Viv Smythe in 2008 as a "a deliberately technically neutral description".
Critics of the acronym claim it is an abusive term, while New Statesman author Sarah Ditum claimed that since its inception the term "functioned to blame feminism for male violence against trans people."
Many fans of JK Rowling and the Harry potter franchise have tweeted their disappointment at the author's post.
Fellow author Sophia Elaine Hanson commented: "JK Rowling gave the world an incredible gift with Harry Potter. That does not mean she is above criticism or justifiable anger. She has proven time and time again that she is not only transphobic, but unwilling to unlearn her prejudices."
Another fan said: "i’m extremely disappointed. i grew up adoring your books and looking up to you. i wanted to be an author, (i still do) and you were my idol. but i cannot stand for this. why should the way a trans person lives their life matter AT ALL to you? #TransRightsAreHumanRights"
LGBTQ advocate Amanda Jette Knox also tweeted her disappointment, stating: "My daughter, who is trans, is a big fan of yours. It breaks my heart to see you post something indicating that discrimination against her is perfectly fine behaviour for an employee. The world’s most credible medical orgs affirm trans people. Please catch up."
Some have defended Rowling, with author Stephanie Davies-Arai replying "Huge thanks to you for speaking out."