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.
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.
