“OMG, there he is. We love you, Matty!” screamed hordes of adolescent girls waiting in line to meet their favorite music artist — and, likely, crush — the up-and-coming rap singer Matthew Morris, otherwise known as MattyB.

The 13-year-old YouTube sensation greeted approximately 100 excited young fans and their family members at a book signing at the Boston University Barnes and Noble bookstore Saturday afternoon to promote his autobiography, “That’s A Rap,” which is the No. 1 best-selling book in Amazon’s Children’s Rap and Hip-Hop section.

Jumping up and down in anticipation, fans, wearing homemade T-shirts with Morris’s face on them, bought the $19.99 hardcover editions of his book at the store before meeting the internet sensation, who sat at a table.

Kayleigh Munglivan, 12, of Westford, Massachusetts, was accompanied by her parents. They arrived two hours early to be the first in line to meet MattyB. Kayleigh said she was so excited that she almost fainted after getting Morris’s autograph.

“I was crying a little bit,” she said. “He came by the line and high-fived my hand, so I’m definitely not going to wash this hand again.”

Fans took photos with Morris while he autographed the books. While security did not allow fans to bring cell phones or bags when they met Morris, they brought gifts for the singer to autograph. Some carried glittery, colorful cards made just for Morris.

Erin Cropper, 12, of Hampton, New Hampshire also stood at the front of the line with her parents. Cropper said she became a MattyB fan after listening to one of his songs that featured another YouTube sensation, Cimorelli.

“My friend showed me his cover of ‘Call Me Maybe’ with Cimorelli a while ago,” she said. “I was very intrigued, and I’ve been watching his videos ever since. [Meeting Morris] was amazing. I’m just in shock … this is really the best day of my life.”

Morris’ fame began when he started posting videos on YouTube at age 7 in 2010. Since then, his notoriety has skyrocketed. He was one of the top 75 most viewed YouTube artists last year, gaining more than 90 million average monthly views, according to his website.

Some fans said Saturday was not the first time they had met Morris in person. Kaitlin Kramer, 19, of Ayer, Massachusetts said she met Morris after buying VIP tickets for one of his concerts in Boston in 2014.

“The first time [I met him] I just said ‘Hi’ and couldn’t really talk,” she said. “But I was definitely more confident going in this time.”

Kara Kramer, Kaitlin’s mother, said she likes Morris because he makes “clean” songs that are appropriate for children and teens.

“He takes [music] and cleans it up so the kids can sing it on the playground” she said. “And it’s fine [with me].”

Kara Kramer said Morris’ remix cover of the Cyndi Lauper song “True Colors,” featuring singer Olivia Kay, is an inspiration.

The video, which boasts more than 70 million views on YouTube, also features his younger sister, Sarah Grace, who has down syndrome.

“It is so adorable how much love he has for his little sister,” Kara Kramer said. “I have a little girl with cerebral palsy, so that’s a big thing for me.”