Pul, who had led a revolt against the leadership of former Chief Minister Nabam Tuki, was administered the oath office and secrecy by Governor K P Rajkhowa at a ceremony at the Raj Bhawan in Itanagar.
Pul, 48, heads a government that has the support of 19 dissident Congress MLAs, 11 of BJP from outside and two independents, who are likely to be part of the government, in the 60-member state assembly.
Tuki commands the support of 26 Congress legislators.
After the oath-taking, Pul told the media that he would expand his ministry after consultations with the MLAs backing him but did not specify any date for the exercise.
Speculations are rife about the possibility of dissolution of the assembly by the new government to pave the way for fresh elections in the frontier state.
Earlier in the day, with the Supreme Court paving the way for government-formation, President's rule in Arunachal Pradesh was lifted.
President Pranab Mukherjee gave his approval to the Union Cabinet's recommendation for revocation of President's rule, a home ministry official said in New Delhi.
The President's nod came after a last-ditch effort of Nabam Tuki seeking an interim direction that his party be allowed to go for a floor test in the assembly to prove majority was rejected by the Supreme Court on Friday.
The Union Cabinet had recommended the withdrawal of President's rule in Arunachal Pradesh on Wednesday after Pul, accompanied by 31 MLAs, including Congress rebels, BJP and independents had met Rajkhowa and staked claim to form government.
A revolt by Congress dissidents led by Pul triggered political crisis in the state that finally led to imposition of President's rule on January 26.
Congress, which had 47 MLAs in the 60-member assembly, suffered a jolt when 21 of its lawmakers revolted against Tuki's leadership. Two of them later resigned from the House.
Later, 14 rebel Congress MLAs were disqualified by the Speaker.
The Supreme Court is considering pleas against imposition of President's rule in Arunachal Pradesh and also hearing petitions seeking examination of constitutional schemes on the scope of discretionary powers of the Governor.
However, on Thursday, the apex court virtually paved the way for government formation in Arunachal Pradesh by vacating its order on maintenance of status quo, after being satisfied with the Gauhati High Court order staying the disqualification of the 14 rebel Congress MLAs.
A former Finance and Health minister of the state, Pul represents Hayuliang assembly constituency and was elected as the leader of the House and the chief minister by the rebel Congress MLAs, BJP MLAs and two independents in the assembly session which had been held at an auditorium as a make-shift venue on December 17. The session was boycotted by supporters of Tuki after whose ouster President's rule had been imposed.
When his attention was drawn to the claim of Tuki that he still enjoyed the status of CLP leader and majority in the House with 26 MLAs, Pul said that it is inner-party democracy and the new government is a purely Congress government with outside support of BJP members and two independent.
"Congress is a big party... There may be differences in opinions... I worked with Tuki. But whatever grievances we raised in CLP meeting, nothing was redressed. BJP, despite being an opposition party, too wants development," he said.
Pul said the priority of his government would be to improve the law and order situation and to streamline education, health and power sectors besides improvement in delivery system of essential services to the people without any discrimination."
"Improvement in communication besides infrastructure development in the state will be other priority areas of my government," he added.
Lauding the people of the state for reposing faith in him, Pul claimed that the people are with him because they have realised the genuine cause for which he was fighting.
A revolt by Congress dissidents led by Pul led to a political crisis in the state that finally led to imposition of President's rule on January 26.
The Supreme Court is considering pleas against imposition of President's rule in Arunachal Pradesh and also hearing petitions seeking examination of constitutional schemes on the scope of discretionary powers of the Governor.
After the Centre's recommendation for withdrawal of President's rule, the Congress moved to the Supreme Court and got an order for maintenance of status quo in the crisis-ridden state till it examined judicial and assembly records on disqualification of 14 rebel Congress MLAs by former Speaker Nabam Rebia.
However, on Thursday, the Apex court virtually paved the way for government formation in Arunachal Pradesh by vacating its order on maintenance of status quo, after being satisfied with the Gauhati High Court order staying the disqualification of the 14 rebel Congress MLAs.
Congress again moved the Supreme Court on Friday but the Supreme Court rejected its plea for an interim direction that it be allowed to go in for a floor test in the Arunachal Pradesh assembly to prove majority.