Xi has already been elected as general secretary of the ruling Communist Party of China (CPC) at its once-in-five-year congress in October last year.
Xi and Wang took the oath of office by pledging allegiance to the Constitution, into which the president's political thought was incorporated as the amendment of the country's supreme law was enacted for the first time since 2004 earlier this month.
The National People's Congress, China's rubber-stamp legislature, also appointed close Xi ally Wang Qishan to the formerly ceremonial post of vice president. Only one delegate voted against Wang, out of the total 2,969.
Mr Xi, 64, is considered the most powerful Chinese leader since Mao Zedong.
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Wang, who previous year retired from China's elite Standing Committee which rules the country, spearheaded Xi's anti-corruption campaign, which has netted millions including high flying politicians for graft.
Wang's appointment shows that "he's a really important political advisor", said Kerry Brown, director of the Lau China Institute at King's College London.
The oath also stipulates that they will be committed and honest while performing duties, accept people's supervision and work hard to help build a great modern socialist country that is prosperous, strong, democratic, culturally advanced, harmonious and handsome.
Last Sunday, parliament voted to amend the constitution, which removed presidential and vice-presidential term limits, meaning Xi can stay in power indefinitely.
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Saturday's plenary session also saw the approval of a wide ranging series of reforms of the country's government, which will feature restructuring and changes to 11 ministries. He first became president in 2013. Critics point out that the election of Wang too was in violation of the unwritten rule of retirement at age 68 followed by all top CPC leaders so far.
Over the next two days, the congress is due to approve the head of that new anticorruption commission as well as other senior government appointments, including the posts of premier and central-bank governor.
Wang is said to have Xi's eyes and ears.
Mr. Wang's formal return to front-line politics and his emergence as a leading figure in China-U.S. relations comes as Washington rolls out punitive measures to address a bilateral trade imbalance that a year ago stood at $375 billion in China's favor.
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From India's perspective of the new line-up of officials, Chinese Foreign Minister Wang Yi is widely expected to be elevated as state councillor which makes him the top diplomat of the country. The post is now held by Yang Jiechi who has been elevated to the politburo, the top policy body of the ruling Communist Party of China (CPC).