The Library of Congress is to announce Thursday that the next poet laureate is Natasha Trethewey, the Pulitzer Prize-winning author of three collections and a professor of creative writing at Emory University in Atlanta. Ms. Trethewey, 46, was born in Gulfport, Miss., and is the first Southerner to hold the post since Robert Penn Warren, the original laureate, and the first African-American since Rita Dove in 1993.

“I’m still a little in disbelief,” Ms. Trethewey said on Monday.

Unlike the recent laureates W. S. Merwin and her immediate predecessor, Philip Levine, both in their 80s when appointed, Ms. Trethewey, who will officially take up her duties in September, is still in midcareer and not well-known outside poetry circles. Her work combines free verse with more traditional forms like the sonnet and the villanelle to explore memory and the racial legacy of America. Her fourth collection, “Thrall,” is scheduled to appear in the fall. She is also the author of a 2010 nonfiction book, “Beyond Katrina: A Meditation on the Mississippi Gulf Coast.”

“The appointment of Natasha Trethewey is a very welcome event,” said Dana Gioia, a former chairman of the National Endowment for the Arts and an early admirer of her work. “She writes out of the complicated history of the region, and even from her own complicated history.” In a phone interview explaining his choice James Billington, the librarian of Congress, said: “We’re not necessarily on some kick to find a younger poet. The more I read of it, American poetry seems extremely rich in diversity, talent and freedom of expression, and she has a voice that is already original and accomplished. I have an affinity for American individuals who are absolutely unique, and I think that this is one.”

He first became aware of Ms. Trethewey (pronounced TRETH-eh-way) when she gave a reading at the National Book Festival in 2004: “I admired the way she had a certain classical sound but also moved easily from traditional forms to free verse. And then when I began reading her poems for myself, that impression was just confirmed. It seemed very natural, all of a piece.” He added: “I go to a fair number of poetry readings, and I’m not always motivated to go back and read the poems. But in her case I was.”