The title is based on a line from the poem "Kabul" by Iranian poet Saib Tabrizi, as translated by Josephine Davis. The poem is recited by Babi when Laila's family is preparing to leave Kabul. Originally, Hosseini had just been looking for a poem for that scene, but, according to an interview with Book Browse, he found the phrase "a thousand splendid suns" to be an "evocative title."
The phrase also shows up when Laila is teaching at the orphanage at the end of the novel. Laila is comforted by the realization that "Mariam is in Laila's own heart, where she shines with the bursting radiance of a thousand suns" (4.51.37). Fittingly, the title becomes a reference both to the city of Kabul and to the women of Afghanistan—the two subjects that the novel is most interested in.