Meloni, head of the nationalist Brothers of Italy, swept to victory in an election last month as part of a coalition that included Forza Italia, led by former premier Silvio Berlusconi, and Matteo Salvini's League.
"I swear to be faithful to the republic," the 45-year-old Meloni said on Saturday under the crystal chandeliers of a frescoed chamber, before shaking hands with President Sergio Mattarella.
Her government, the 12th this century, replaces a national unity administration led by former European Central Bank chief Mario Draghi.
It faces a string of daunting challenges, notably a looming recession, rising energy bills and how to present a united front over the Ukraine war.
While Meloni has pledged support for Ukraine, Berlusconi has repeatedly undercut her, earlier this week blaming Kyiv for the Russian invasion and revealing he had exchanged gifts and "sweet letters" with his old friend, Russian President Vladimir Putin.
After days of often tense, behind-the-scenes talks, Meloni unveiled her team on Friday, giving five ministries each to her junior partners, the League and Forza Italia, while reserving nine cabinet posts for her own party.
Technocrats make up the rest of the 24-strong squad, which includes just six women.
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