Renzi has sketched out an ambitious agenda, promising to tackle electoral and constitutional reform, make the labour market and tax systems more efficient and overhaul the bloated public administration all within four months.
On Friday, ratings agency DBRS said the reforms would be positive for Italy as long as the "ambitious timetable" was respected.
However, as well as having to deal with the same unwieldy coalition which hampered Letta, Renzi will also face questions about how he gained office, which could limit his ability to push through unpopular reform measures.
At 39, he would be Italy's youngest-ever prime minister, but he would also be the third in a row to gain office without winning an election and opinion polls suggest many Italians are concerned about the lack of a mandate from voters.
Angelino Alfano, head of the small center-right NCD party which Renzi will depend on for a majority, is expected to keep his post as interior minister but will no longer have the title deputy prime minister after Renzi ruled out giving him a post that could challenge his own authority.
(Read More: Renzi set to become Italy's youngest prime minister)
The NCD is also expected to hold on to the transport and health ministries, both of which it held under Letta.
As head of the OECD's economics department, Padoan has called for aggressive easing from the European Central Bank and was an early critic of tough budget cutbacks in the euro zone's weakest economies as they struggled with excessive debt.
A poll on Friday by the SWG polling institute posted a dip in support for the PD, to 29.9 percent from 32.2 percent a week earlier, while support for former Prime Minister Silvio Berlusconi's Forza Italia rose to 21.8 percent from 20 percent.
The survey showed 27 percent saw Renzi as a leader capable of giving a future to Italy, more than any other potential rival on the list but still outscored by the 30 percent who picked "none".
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