No casualties were reported and firefighters were able to contain the blaze at the Tawar Mall, according to the Ministry of Interior and the mall operator.
Photos on social media showed thick black smoke billowing from the complex, which is due to open in the last three months of this year with more than 300 retail outlets.
The mall said the fire was concentrated in "a small area" on the second floor. The company's chairman, Jassim al-Kuwari, said it caused minimal damage and would not delay construction.
In April, a fire broke out at another mall being built in the capital of Doha. That blaze near a stadium that will host some of the World Cup forced the evacuation of 14,000 workers. A 2012 fire at yet another shopping mall, the high-end Villaggio, was the country's deadliest disaster in recent memory. That fire killed 19 people, including 13 children, many of them foreigners.