Before her Sweet Sixteen, Vanessa Trump sent her high school sweetheart — then a member of a notorious street gang who was just beginning a 16-month prison sentence for assault — a love note saying she dreamed of having children with him.

Page Six has exclusively revealed that despite her being a quiet member of America’s first family and the estranged wife of Donald Trump Jr., Vanessa had a first true love: Valentin Rivera, a member of the violent Latin Kings street gang.

She dated Rivera for five years from when they were just 15, and her letters reveal she couldn’t wait to start a family with him at just 18 years old.

In a note dated Dec. 13, 1993 — packed into an envelope labeled SWAK (“Sealed With A Kiss”) written on the back — Vanessa wrote, “I miss you a lot, especially since it’s my birthday coming up. And I wish you were here to celebrate with me. But you’ll be out [of prison] for my 18th birthday.”

She added in the note — one of nearly a dozen prison letters handwritten by Vanessa to send to Rivera and seen by Page Six — “I can’t wait till that year because a lot’s going to happen. My 18th birthday, you’ll be back in my arms, my prom, and I want to get pregnant and have a baby with you after January. I want to get pregnant so the baby is born after I graduate since it takes 9 months.”

In an interview, Rivera told us that Vanessa — whom he met when they were in elementary school at P.S. 158 on the Upper East Side — wrote to him almost every day during his stretch for assault, during which he joined the infamous Latin Kings gang.

Her notes were littered with the code “831,” meaning “I love you” (meaning: eight letters, three words, one meaning) and always with the return address of her upscale Sutton Place home. She stuck with Rivera despite her privileged upbringing provided by her mother, Bonnie Haydon, the owner of a modeling agency, and her late stepfather, Charles Haydon, a famed Manhattan attorney.

But Vanessa — who became a student at $47,000-a-year Upper West Side prep school Dwight — also became increasingly attracted to the gang lifestyle, according to Rivera.

And in the letters she wrote to him while he was inside, she described her own flirtation with thuggish behavior.

“Lately people in my bitch ass school are acting like bitches, and today I flipped,” she wrote, “I went up to shorty, Jen, and confronted her about something.”

Vanessa describes how an argument between the two girls escalated into a fight, and, she says, “I pushed her and our hands were together pushing each other so I slammed her against the wall holding her against the wall, screaming at her.”

Even though she was never a gang member herself, Vanessa intimates she could have been developing a reputation because of her relationship with Rivera. “People honestly were scared for her, because … people have heard about me, and they didn’t want any s–t happening.”

But of his own gang activity, Vanessa — who sometimes addresses Rivera as “King” in the messages — warns him, “So if you are ‘L.K.’ [Latin King] just remember please stay out of trouble and just chill and be good because no one loves you as much as I do.”

But she also seems jealous of his brotherhood with other members of the gang, founded in Chicago in the ’40s and spread to New York prisons in the ’80s.

“When we get in conversations about the ‘Latin Kings’ I just want you to act the same for me,” she writes, “and always be there for me because I obviously don’t want to lose you, I just want you to do the best you can and stay out of trouble because I can’t go through you getting locked up [again].”

But her letters are also often heated, offering a glimpse at the young couple’s difficult circumstances. In one message approximately six months into his term in an upstate prison, seemingly in a moment of frustration and despair, Vanessa even tried — unconvincingly — to break up with him.

“I know it’s tough for you up there and I don’t do anything to help the situation. And I love you and I want you more than just a phone call,” she writes, “I want to be able to hold you and make love to you, but that can’t happen. So my decision is for the best and only the best, but whenever you need someone to talk to or if you need clothes or anything, I’m here, just call.”

After several more stints in prison, Rivera — who ended his relationship with Vanessa when she was around 20, after discovering that she had been unfaithful to him with actor Leo DiCaprio, whom she met in a New York Club — left the gang for good.

Vanessa went on to meet Don Jr. in 2003 and married him in 2005 at his father’s Mar-a-Lago estate in Palm Beach, Florida. After having five children with Don Jr., she filed for divorce in March this year.

Rivera has turned his back on his gangland past and now has a steady job and a family. But, even though he has moved on with his life, he says he has never been able to part with his love letters from Vanessa.

Rivera explained, “Vanessa was my first love, she even gave me a photo album full of pictures of her, and us together, with a note saying, ‘This is so you can remember me when I become famous.'”

“You know, she was right, she did become famous — but the Vanessa people see with Trump isn’t the Vanessa I knew.

“She always wanted to become a mother, but she was also a much stronger, independent and fiery woman. She had big dreams, she’s a fighter.

“I hope she finds happiness in the next chapter of her life.”

When reached for comment, a spokeswoman for Vanessa told us, “Vanessa is a devoted mother of five amazing children. This is nothing more than an attempt to sensationalize over 20-year-old ‘stories’ from high school for clickbait.”