On Thursday night, in a basement strewn with garbage under a former furniture warehouse in Bushwick, Yung Lean, a baby-faced seventeen-year-old rapper from Stockholm, ran up behind Yung Gud, an eighteen-year-old friend who makes his beats, and hugged him for a few seconds before they both started laughing. A few hours later, Yung Lean would play a concert in the space upstairs. Although Yung Lean is almost totally unknown in Sweden, his music, a stream of near-monotone observations and declarations focussed on his own social discomfort and rapped in the vernacular of American Top Forty rap (“I’m nervous that she’s upset / She don’t want me / Bitch, I’m flustered”), has drawn enough of a following in the United States to support a nationwide tour. Videos for several of Yung Lean’s most popular songs have more than two million YouTube views. On Wednesday and Thursday, Yung Lean played three sold-out concerts in New York, his first appearances in the United States.

“America is so much more ‘show business.’ For instance, you have Barack Obama. We have Fredrik Reinfeldt. Everyone in the world knows Barack Obama! In the U.S., everything is big—it’s like looking through a magnifying glass,” said Yung Lean, sitting on the street in SoHo a few hours earlier, looking up with curiosity at the buildings. “I’ve always been an outsider everywhere I go—I don’t fit in with the Swedish rap community or the American rap community. But who cares?” He was about to play an in-store concert at VFiles, an Internet-centric fashion retailer in SoHo. Heidi Mount, a supermodel who succeeded Claudia Schiffer as the face of Chanel in 2009, guarded the door with a clipboard, regretfully informing shoppers that there was an invite-only event happening in the store and that they could not enter.

On the steps behind her stood a gang of Yung Lean’s teen-age fans, high-school students who intern at fashion labels in SoHo. “Yung Lean, he’s different. There’s not a lot of people who bring what he brings to the industry. I relate to a lot of the things he’s saying: clothes, weed, ‘lean,’ ” said Roger, a fifteen-year-old intern at VFiles and a student at NYC iSchool, a humanities-focussed public high school in SoHo. (“Lean” is a fixture in American rap—it’s a beverage comprised of Sprite, one Jolly Rancher hard candy, and Promethazine cough syrup with codeine. Yung Lean’s name is a play on his middle name, Leandoer, and “lean.”) Roger continued, with a sly smile revealing his braces, “Even though I don’t do any drugs, I can still relate to it.”

Miles, a fifteen-year-old intern at Been Trill, another New York fashion label (who also sported braces), chimed in, “Even if you don’t understand what he’s saying, it’s about a feeling—a really heavy feeling.” A generation ago, Yung Lean might have cooed over a fingerpicked guitar, but today his medium is confessional rapping over airy, melancholy rap beats. Preston Chaunsumlit, the thirty-four-year-old star of a popular YouTube series produced by VFiles, explained, “I’m from a generation where you see a rapper who’s a dinky little white kid and it’s ironic, but for this generation it’s not like that.”

A listener doesn’t need to understand every one of Yung Lean’s lyrics to understand what he’s generally trying to tell the listener: that he’s sad. In the vein of rap crews of earlier eras who sought to communicate toughness (Ruff Ryders, N.W.A., Murder, Inc.), newfound status (Cash Money Records, Junior M.A.F.I.A.), or a political stance (Native Tongues Posse, Zulu Nation) through their names, Yung Lean’s crew communicates melancholy—they’re called Sad Boys. (A Virginia rapper who goes by the name Lil Ugly Mane works in a similar space; his most popular song is called “Bitch I’m Lugubrious.” Little Pain, a Brooklyn rapper, does the same—his signature track, “High Cry,” features the memorable couplet, “I’m smokin’ while I’m cryin’ / I’m cryin’ while I’m rhymin’ / and if I had a whip I’d be cryin’ while I’m drivin’.”) Indeed, in the course of Thursday evening Yung Lean and his crew seemed self-conscious about smiling, occasionally catching themselves and reverting to gloomy looks when they realized an onlooker was watching.

In Yung Lean’s best music video, for the song “Yoshi City,” he sits on the hood of a Smart Car with open scissor doors, repeating either, “I’m a lonely clown with my windows down,” or, “I’m on my lonely cloud with my windows down,” trying to suppress a smile. In one shot, the video appears to cut away when Yung Lean’s smile threatens to overpower his face. Yung Lean can’t help but put two prominent sensations associated with teen-agers on display: angst and the joy of discovery. In other videos, like “Motorola,” Yung Lean’s cherubic face conveys blankness, boredom, and the frigidity of his hometown—his nose is bright red in the Stockholm air.

In person, Yung Lean is polite and guarded, if not funereal, preferring to let emissaries like Yung Gud, his producer, speak for him. In the Bushwick basement, sitting beside Yung Lean, Yung Gud described their relationship: “I work differently with other artists because it’s more of a business relationship, more of a professional relationship. But with us it’s like kindergarten! We do whatever we want. When we first met each other, all we wanted to do was wear morphsuits,” he said, referring to the skintight spandex suits that cover the entire body. “Yeah,” said Yung Lean. “The first time we met, I said, ‘Yung Gud, we gotta wear morphsuits!’ ”

Yung Lean grabbed an empty drum of paint and sat down on it, toting a bottle of over-the-counter cough syrup that he sipped and shared with Yung Gud. (Yung Lean noted that he was losing his voice.) “Mainly, what we do is go into the studio and then do this—just hang out,” said Yung Lean, looking back at Yung Gud expectantly, waiting for him to continue. Yung Gud sat silently, and so Yung Lean, in the thrall of discovery, again at odds with Sad Boys’ aesthetic, picked the conversation back up. “You can hear our music evolving,” he said. “We do a track and say, ‘This is better than what we did an hour ago, so much better than what we did two weeks ago!’ ”

This prompted Yung Gud to describe the process of recording two of their current hits. He said, “We made ‘Kyoto’ in ten or fifteen minutes. It was just one take, and we built the chorus out of that. ‘Ginseng Strip 2002’ was just a sound check—he was just checking to see if the microphone was working.” He reflected for a moment, then added, “The first idea is always the best one.” Yung Lean cut him off: “And then you always want to return to the first idea,” he said. “The first idea is always the best.”

On his American tour and his forthcoming début album, “Unknown Memory,” Yung Lean will put his American audience’s appetite for his musical evolution, if not morphsuits, to the test. Emilio Fagone, a Stockholm club promoter who is also Yung Lean’s manager, seemed optimistic. “A lot of people in Sweden are having a hard time dealing with it,” he said. “I guess they don’t understand it. But I would say they’re the Ace of Base of 2014—nobody listened to them in Sweden, but they were huge here. They met President Clinton! I think it’s going to be the same thing with Sad Boys.”