Hizbul Islam spokesman Mohamed Moallin made the surprising announcement on Monday, the Press TV correspondent in Mogadishu reported.
Moallin told reporters that al-Shabab fighters are no different than unbelievers.
He went on to say al-Shabab has misinterpreted Islam and Hizbul Islam is ready to hold talks with the new government.
Hizbul Islam also announced that it only has one condition for joining the new government and that condition is that it should be an Islamic government.
In late 2010, Hizbul Islam merged its forces with al-Shabab.
However, the leader of Hizbul Islam, Sheikh Hassan Dahir Aweys, later censured al-Shabab for wantonly killing civilians and for its connections with al-Qaeda.
In March 2012, Sheikh Hassan said, “I warn my jihadist brothers in al-Shabab against [spilling] the blood of the Somali nation and killing innocent civilians in the name of Islam."
Late on Saturday, al-Shabab militants said they assassinated a member of Somalia’s new parliament in Mogadishu and threatened to kill every MP in the country.
Gunmen shot and fatally wounded MP Mustaf Haji Mohamed in the Waberi district of the capital on Saturday. He was rushed to a local hospital but died on the way.
Mustaf was the father-in-law of former President Sheikh Sharif Sheikh Ahmed.
“How many times had we warned Somalis against joining the infidel government?” a spokesman for the group said afterwards. “Let it be a good lesson for the rest.”
The new Somali parliament was sworn in on August 20.
Somalia has not had an effective central government since 1991, when warlords overthrew former dictator Mohamed Siad Barre.
However, MPs meeting in Mogadishu elected Hassan Sheikh Mohamud as the new president of Somalia with a big majority on September 10.
The weak Western-backed transitional government in Mogadishu has been battling al-Shabab fighters for more than five years and is propped up by a 10,000-strong African Union force from Uganda, Burundi, and Djibouti.