It said the girl's mother had filed the divorce case with the court at Unayzah 220km north of Riyadh, and cited lawyer Abdullah Jtili as saying the father had arranged the marriage without telling the girl.
"She doesn't know yet that she has been married,'' added Jtili of the girl who is about to begin her fourth year at primary school.
Al-Watan said relatives of the girl had told a Saudi rights group of her plight and urged it to intervene to have the marriage annulled.
But the daily also reported that the husband had refused to renounce the marriage, saying that he had not done anything illegal.
Arranged marriages involving pre-adolescents are occasionally reported in the Arabian Peninsula, including in the ultra-conservative Saudi kingdom where the strict conservative Wahabi version of Sunni Islam holds sway and polygamy is common.
In Yemen in April, another girl aged eight was granted a divorce after her unemployed father forced her to marry a man of 28.