Warsame Mohamed Hassan, a deputy mayor in Mogadishu, told the United Nations' humanitarian news agency IRIN that former residents were returning in droves.
"We estimate that about 1 million people have returned to Mogadishu this year," he said.
Al-Shabaab, a militant group allied with al-Qaida, has lost ground in Somalia as the interim administration in Mogadishu gains a foothold in the country. African forces in the country have fought alongside pro-government troops to restore security to the capital.
Hassan said the population in Mogadishu may be close to 3 million people, twice the estimate from July 2011.
Somalia has by members of the international community as an emerging success story. Oxfam, however, warns there's no room for complacency in a country that hasn't had a functioning central government since the 1990s.
"Somalia needs more than a band aid," Oxfam's Somalia Director Senait Gebregziabher said in a statement.