Robert De Niro is supporting his daughter after she came out as transgender … telling TMZ she has his full love and support.

The legendary actor tells us … “I loved and supported Aaron as my son, and now I love and support Airyn as my daughter.”

Robert adds … “I don’t know what the big deal is … I love all my children.”

As we reported … Airyn came out as a transgender woman this week in an interview with Them — an online LGBTQ+ media outlet — telling the world she’s “stepping into this new identity.”

Robert has 7 kids … and Airyn was born Aaron with her twin brother, Julian. Their mother is actress Toukie Smith.

Airyn also opened up about struggling with body dysmorphia, a mental health condition where someone obsesses over flaws in their appearance.

She said growing up, she was told she was “too big, not skinny enough, not Black enough, not white enough, too feminine, not masculine enough” and adds, “It was never just, ‘You’re just right, just the way you are.’”