Commanders say the Islamist Al-Shabaab group abandoned the town without a fight as their troops approached.
Despite facing pressure on a number of military fronts, the al-Qaeda group still controls much of the country.
"Hopefully the next target will be Kismayo and then we will proceed to other towns and cities," interim Prime Minister Abdiweli Mohamed Ali told the BBC's Focus on Africa programme.
"Surely but slowly we're getting our country back from Al-Shabaab," he said.
'No military muscle'
Somalia analyst Mohammed Abdulahi Hassan told the BBC its importance lies in that fact that a network of roads from the town leads all over the country.
He said the militants were unlikely to have the "military muscle to retake the town from the Kenya defence forces".
This is the date agreed by disparate Somali factions to elect a new president, ending a transitional period and the mandate of the UN-backed interim government.