Salva Kiir Mayardit, the Sudanese first vice-president and president of southern Sudan, signed the document in Juba, two days after the official arrival of two full battalions of southern troops in the region's capital.
It was signed in the presence of thousands of people, and copies were given to Chief Justice Ambrose Riink and James Wani Igga, the speaker of the southern assembly. Additional copies were sent to the ministry of justice in the Sudanese capital, Khartoum.
The CPA was signed on 9 January by the Sudanese government and the southern Sudan People's Liberation Movement/Army (SPLM/A).
It also represents an important step in fulfilling the southerners' aspirations for greater political autonomy and the decentralisation of power, for which the SPLM/A fought during a 21-year civil war that claimed two million lives.
"This is the first time since independence that the south has got its own constitution," said a political analyst, who declined to be named, on Tuesday.
"It is not just the document; it is the contents," he added. "It will be the foundation of the new Sudan. It spells out people's rights and protections. The government and the president will be held to account using the constitution, and it contains important provisions with regard to equal rights for women and media freedom."
The analyst said that the next step in the implementation of the CPA would be to develop the state constitutions as well as to finalise the setting up of state governments and legislative assemblies.
"It is important to have the state executive and legislative structures up and running as soon as possible so that local issues on the ground can be addressed," he noted.
In a related development, thousands of SPLA soldiers, coming from the southern town of Yei, entered Juba on Saturday during a festive parade. The SPLA troops will form so-called joint integrated units with the northern Sudanese Armed Forces (SAF), as foreseen under the CPA.
"Thousands of soldiers marched into town, with their armour, tanks, artillery, rocket launchers, large trucks, and technicals with mounted heavy guns," said a local observer.
"It was the biggest display of SPLA forces in Juba so far and they were really claiming the town," he added.
One battalion would stay in Juba while another was scheduled to continue to Torit, where Ugandan rebels of the Lord's Resistance Army have been terrorising the region.
"The whole area around Torit is problematic, very tense, so they [the southern government] want to move their troops down there as soon as possible," the observer noted.
Community and political leaders, including Clement Wani, the governor of Central Equatoria State, had called on the local community to welcome the SPLA forces without igniting tensions by displaying hostilities towards the SAF and other former adversaries who still reside in Juba.
"Although the soldiers were singing their SPLA battle songs when they entered town, it was a very disciplined procession and commanders kept a tight lead on their troops. Not a single shot was fired into the air," the observer said.
"After the parade, the crowd - consisting of all kinds of Sudanese communities - broke up and started dancing, wearing traditional dress and waving banners and flags," he added. "They were soon joined by the soldiers and everybody was shouting with relief, excitement and joy."