"The position of the Croatian government and parliamentary majority has been continually clear and firm - we support Finland's ambition and believe that, if that is what our EU partner wants, its choice should be respected and supported," Plenković said at a news conference he held with the visiting Finnish PM.
"We believe that at this moment NATO membership is the security umbrella both Finland and Sweden want," he added.
Finland and Sweden have applied for NATO membership in response to the Russian invasion of Ukraine, which started on 24 February, but their decision has been met with opposition from Turkey which says that Helsinki and Stockholm support Kurdish militants and have imposed an arms embargo on Turkey.
Croatian President Zoran Milanović has been insisting for some time that Finland and Sweden's NATO entry be made conditional on the reform of Bosnia and Herzegovina's election law.
There is no closer relationship than membership in the same defence alliance and readiness to defend one another unconditionally, Marin said.
She added that she believed that NATO would send a strong message of unity at a summit in Madrid next week, stressing that Finland and Sweden would make NATO stronger.