Police in the town of Maiduguri, Borno state, said the attack occurred when the market was full of buyers.
The girls detonated their explosive minutes apart, witnesses said. Both were killed.
No group has said it was behind the bombing, but militants in Boko Haram have carried out similar attacks.
In recent months, the Nigerian army has made gains against the group, but still makes regular bombings.
A Maiduguri militia member, Abdulkarim Jabo, told the media the girls were about seven or eight years old and had hit the market in a rickshaw.
"They left a rickshaw and walked right in front of me without showing the slightest sign of emotion," he said.
"I tried to talk to one of them, in Hausa and in English, but they did not respond, I thought they were looking for their mother."
He said the first girl had headed for a market stall and then detonated her explosive belt.
Militants have carried out a series of deadly attacks in northeastern Nigeria in recent weeks.
On Friday, a double suicide attack by female bombers killed at least 45 people and injured 33 people in a market in the city of Madagali.
In October, suicide bombers also killed 17 people in an IDP camp in Maiduguri.Boko Haram
Founded in 2002, initially focused on opposing Western-style education - Boko Haram means "Western education is forbidden" in the Hausa language
Launch of military operations in 2009
It has killed thousands, mostly in northeastern Nigeria, and kidnapped hundreds, including at least 200 schoolgirls
It joined the so-called Islamic State, now called itself "West African province"
Large area seized in the northeast, where he declared caliphate
The regional force has taken over most of the territory since last year
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