The former Greater Manchester mayor took the oath after newly elected SNP MP Lara Bird and Conservative MP Douglas Lumsden before making his way through a packed Commons chamber.
Burnham repeatedly bowed his head as colleagues applauded his arrival and exchanged a friendly greeting with Speaker Lindsay Hoyle before officially taking his seat as Makerfield's representative.
According to reports from Westminster, he was later met by hundreds of Labour MPs in Westminster Hall, where supporters celebrated what many now view as his imminent rise to the premiership.
Earlier in the day, Burnham confirmed he would stand to succeed Starmer following the prime minister's resignation announcement.
His candidacy immediately received a major boost after former Health Secretary Wes Streeting publicly endorsed him and ruled himself out of the contest.
Streeting had previously been viewed as a potential leadership challenger.
The enthusiastic reception Burnham received from Labour MPs has fuelled speculation that the leadership race may effectively be over before it begins, with no significant rival yet emerging.
Labour is expected to open nominations in early July, though Burnham could become leader unopposed if no challenger secures sufficient backing.
Starmer announced his resignation in an emotional address outside 10 Downing Street, saying he no longer believed he was the right person to lead Labour into the next general election.
Thanking his wife and family, he said he now wanted to be "the best dad I can to my beautiful children" as he prepared to leave frontline politics.
Burnham's expected elevation would continue a remarkable period of political instability in Britain. Since 2019, the country has been led by four prime ministers: Boris Johnson, who resigned in 2022 amid a series of scandals; Liz Truss, whose premiership lasted just 49 days; Rishi Sunak, who succeeded her before losing the 2024 general election; and Keir Starmer, whose tenure lasted less than two years.
If Burnham takes office next month, he will become Britain's fifth prime minister in less than seven years and the seventh since the 2016 Brexit referendum.
The rapid succession of leaders has highlighted the political turbulence that has defined Britain in the post-Brexit era, with governments grappling with economic challenges, rising public dissatisfaction and shifting electoral loyalties.
Burnham now appears poised to inherit both the opportunities and the challenges of leading a country still searching for political stability.