The winner of Best of the Best of the prize's first decade the first decade in 2005 was Andrea Levy's Small Island, which won the main title in 2004.
Established in 1996, the Baileys Women's Prize for Fiction - first known as the Orange Prize for Fiction - is awarded to the best novel of the year written by a woman. Baileys took over sponsorship in 2014.
Adichie's novel was chosen by the chairs of the judging panels from the last 10 years. They included the novelist Joanna Trollope, actresses Miranda Richardson and Sheila Hancock and broadcasters Fi Glover, Kirsty Lang and Muriel Gray.
Gray, who chaired the judges the year Adichie won the Baileys Prize, said: "While it's sometimes pompous to call a book 'important', it's appropriate to say it of Half of a Yellow Sun.
"For an author, so young at the time of writing, to have been able to tell a tale of such enormous scale in terms of human suffering and the consequences of hatred and division, whilst also gripping the reader with wholly convincing characters and spell-binding plot, is an astonishing feat."
Half of a Yellow Sun tells the story of the Nigerian war through the eyes of three characters: Olanna, a young woman who abandons her privileged lifestyle to live with her university professor lover, the shy English writer Richard and the poor village boy Ugwu.
The novel was also named as Radio 4's Woman's Hour listeners' Best of the Best in a vote held on the programme's website.
Adichie's other books include Purple Hibiscus from 2003, which won the Commonwealth Writers' Prize.
Her latest work, Americannah, was published in 2013 and won US accolades including the National Book Critics Circle Award for Fiction and The Chicago Tribune Heartland Prize for Fiction.