The European Union member's previous ruling alliance had collapsed in April after only six months in power, raising the prospect of the third general election in less than two years.
Croatia's notoriously shaky politics have been dominated in recent months by serious financial troubles at retail giant Agrokor, the biggest employer in the region.
In April, Plenkovic -- who leads the HDZ party -- sacked four ministers from his junior coalition partner Most for their refusal to back Finance Minister Zdravko Maric, owing to his links with Agrokor where he was a top manager.
Plenkovic, a 47-year-old former diplomat who only came to power in October, then had to find the support of a new parliamentary majority or call a snap election.
He unexpectedly managed to find a new partner in the small liberal HNS party, which analysts said would help bring the HDZ -- known for its nationalist past -- closer to the centre-ground.
Seven new ministers have been appointed, with the education and construction posts going to HNS members. Maric retained his control over the finance ministry.
Each of the seven ministers was backed in a separate vote by 78 MPs in the 151-seat parliament.
Plenkovic said the avoidance of another election demonstrated "political maturity" in the country of 4.2 million people.
He noted recent developments at Agrokor, which on Thursday said it had secured a 480 million-euro ($537-million) loan, allowing some breathing space for restructuring.
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