The writer's work tells the story of a Nigerian woman who moves to the US to pursue a college education.
In 2008, her second novel, Half of a Yellow Sun, won the UK's Orange Prize and Purple Hibiscus was longlisted for the Booker Prize four years earlier.
Other category winners for the US honour included Sheri Fink's book about Hurricane Katrina.
Her account of the patients, staff and families who took shelter in New Orleans' Memorial Hospital during the devastating storm took the non-fiction prize.
Frank Bidart won the poetry section for his collection Metaphysical Dog, while Amy Wilentz was honoured with the autobiography award for her account of journeys to Haiti following the 2010 earthquake in the country. 'It takes an outsider'
For the first time, a special award was given for a debut writer, crossing all categories.
Anthony Marra was honoured with the prize for his novel A Constellation of Vital Phenomena.
Adichie's third novel was also named as one of the New York Times' top 10 books of 2013.
In an interview with the Los Angeles Times, the author said her book drew on her own experiences as an African living in the US, particularly with African Americans.
"I don't know race in the way an African American knows race… Sometimes it takes an outsider to see something about your own reality that you don't," she said.
Her preceding work, Half of a Yellow Sun, is set during the Biafran War of the late 1960s and has been adapted into a forthcoming film starring Chiwetel Ejiofor and Thandie Newton.
The writer is also in the running for the UK's Baileys Women's Prize for Fiction - formerly known as the Orange Prize - for Americanah.
The National Critics Book Prize was first awarded in 1974 and is open to writers of all nationalities whose work has been published in the US.
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