"Today, my unbreakable commitment with Puerto Rico is stronger than ever," Pierluisi told reporters.
Pierluisi, 60, replaces Ricardo Rosselló , who stepped down as governor following weeks of street protests. Rosselló named him Puerto Rico's secretary of state earlier this week, thus placing him next in line to become governor.
Legislators and political experts are questioning Pierluisi's legitimacy as governor. They said he needs approval from the island's House of Representatives and Senate to fill the role.
Before leaving office on Friday, Rosselló said Pierluisi doesn't need to be confirmed to be named governor, citing a 1952 law.
The House voted Friday and confirmed him for the role. The Senate is expected to vote by Wednesday.
In a press conference shortly after being sworn in, Pierluisi acknowledged that some people were questioning his legitimacy as governor and said that he will respect the Senate's final decision.
"If the Senate ratifies my position there should be no more controversy," he said, adding that he is ready to step down in the case that he's not approved by legislators.
Thomas Rivera Schatz, the president of the Senate, tweeted earlier on Friday that legislators will approach the confirmation process "with the deepest sense of responsibility."
If Pierluisi can't be named governor, Justice Secretary Wanda Vázquez Garced will likely become governor.
Vázquez Garced previously said she doesn't want the job but has said she would follow the rule of law.
"If the time comes, we'll assume the responsibility imposed by the Constitution and the law," Vázquez Garced tweeted Thursday.
The new governor remain in office until January 2021.
Who is Pedro Pierluisi?
Pierluisi is a corporate lawyer for the O'Neill & Borges law firm in San Juan. His firm represents the Financial Oversight and Management Board for Puerto Rico -- which Congress created in 2016 to help manage territory's financial crisis. His brother-in-law is the head of the board, known as la Junta on the island.
One of the more popular chants among protesters prior to Rosselló's resignation was "Ricky renuncia y llévate a la Junta" (Ricky resign and take the Junta with you.)
Pierluisi is also Puerto Rico's former resident commissioner, the island's sole representative in Congress, from 2009-2017. He also previously served as Puerto Rico's secretary of justice under former Gov. Pedro Rosselló, the outgoing governor's father.
Rosselló defeated Pierluisi in 2016 when they sought the New Progressive Party nomination for governor. After his loss, Pierluisi moved to the private sector.
In his statement announcing the nomination, Rosselló said Pierluisi's previous positions make him an ideal candidate to confront the current political challenges.
"This historic time requires a person able to re-establish relations with all sectors at the local and national level," said the outgoing governor.
Rosselló has said Pierluisi will finish out his term but will not seek the governorship next year.
San Juan Mayor Carmen Yulín Cruz criticized Rosselló and the confusion of the appointment process.
"Ricardo Rosselló wouldn't stop stealing and took Puerto Rico's right to have a governor who doesn't represent the interests of the board. Today the board swears in. A total insolence!" she tweeted Friday.
Governor signed bill hours before resignation
Shortly before his effective resignation, Rosselló's signed a bill into law to move the Democratic presidential primary election from June to March.
"The law intends to bring national attention to Puerto Rico, especially in the upcoming Democratic primaries. Currently, the primary is to be held in June, which reduces the impact we may have. By making Puerto Rico an early voting state, candidates will be forced to pay attention to our needs," he said in a statement.
Rosselló's historic resignation came after nearly two weeks of mass protests in Old San Juan and in the midst of political scandal involving the release of crude, sexist and homophobic chat messages between the governor and members of his inner circle.
Following the footsteps of his father, former Puerto Rico Gov. Pedro Rosselló, he ran for governor in 2016 and took office in January 2017.
The eventual downfall of his administration may have been set in motion by Hurricane Maria in 2017. The devastating storm made landfall less than nine months after Rosselló became governor.
In the wake of Hurricane Maria there were widespread problems with the distribution of food, water and other vital supplies to those who needed it most. Many residents were left in the dark for months when the island's antiquate power grid was heavily damaged by the storm.
Nearly a year after the storm, the Rosselló administration finally admitted that Hurricane Maria left several thousand people dead -- not the dozens that had been the official line.