Bob Dylan, the poet laureate of the rock era, whose body of work has influenced generations of songwriters and been densely analyzed by fans, critics and academics, was awarded the Nobel Prize in Literature on Thursday.

It is the first time the honour has gone to a musician. In its citation, the Swedish Academy credited Mr. Dylan with “having created new poetic expressions within the great American song tradition.”

The choice of Mr. Dylan for the world’s top literary honour came as something of a surprise and was widely viewed as an expansion of the academy’s traditional notions of art. Mr. Dylan, 75, joins a pantheon that includes T. S. Eliot, Gabriel García Márquez, Samuel Beckett and Toni Morrison — the last American to claim the award, in 1993.

“The old categories of high and low art, they’ve been collapsing for a long time,” said David Hajdu, a music critic for The Nation who has written extensively about Mr. Dylan and his contemporaries, ”but this is it being made official.”

In choosing a popular musician for one of the most coveted prizes in the literary world, the Swedish Academy dramatically redefined the boundaries of literature, setting off a debate about whether song lyrics have the same artistic value as poetry or novels.

“Most song lyrics don’t really hold up without the music, and they aren’t supposed to,” the poet Billy Collins said. “Bob Dylan is in the 2 percent club of songwriters whose lyrics are interesting on the page, even without the harmonica and the guitar and his very distinctive voice. I think he does qualify as poetry.”

In previous years, writers and publishers have grumbled that the academy seems to favor obscure writers with clear political messages over more popular figures — recent winners have included the Swedish poet Tomas Tranströmer and the German novelist Herta Müller. But in choosing someone so well known and commercially successful, and so far outside of established literary traditions, the academy seems to have swung far into the other direction.

It’s not the first time it has stretched the definition of literature. In 1953, Winston Churchill received the prize, in part as recognition of the literary qualities of his soaring political speeches and “brilliant oratory in defending exalted human values,” according to the academy. And many were surprised last year, when the prize went to the Belarussian journalist Svetlana Alexievich, whose deeply reported narratives draw on oral history. Still, Mr. Dylan is perhaps the most radical choice in the 115-year history of the literature prize.

Sara Danius, a literary scholar and the permanent secretary of the 18-member Swedish Academy, which awards the prize, called Mr. Dylan “a great poet in the English-speaking tradition” and compared him to Homer and Sappho, whose work was delivered orally. Asked if the decision to award the prize to a musician signaled a broadening in the definition of literature, Ms. Danius responded, “The times they are a-changing, perhaps,” referencing one of Mr. Dylan’s songs.

The choice was hailed across the cultural and political spectrum. On Twitter, President Obama offered “congratulations to one of my favorite poets,” while Rosanne Cash, the songwriter and daughter of Johnny Cash, wrote simply: “Holy mother of god. Bob Dylan wins the Nobel Prize.”

  Bob Dylan, Music, naked truth, Nobel Prize