The assurance came after a brief late Thursday night telephonic talk Modi had with Shiv Sena President Uddhav Thackeray.
Modi's name is likely to be announced Friday by the Bharatiya Janata Party (BJP) for the country's top post. The Shiv Sena has been sitting on the fence over it and in the past, it had rooted for Sushma Swaraj as PM nominee.
Party spokesperson and MP Sanjay Raut said that the Shiv Sena has decided to support Modi or any other person its ally the BJP puts forth as PM nominee.
However, he refused to disclose the details of the conversation the two leaders had Thursday night.
Interestingly, the Sena's change of heart comes exactly a week after Thackeray had urged the BJP to let Modi remain as Gujarat CM till 2017.
"Narendra Modi has publicly declared that he has no prime ministerial ambitions. He has been given public mandate to rule Gujarat till 2017 and would continue to serve the people of that state," Thackeray had said in an editorial in the party mouthpiece "Saamana" last Saturday.
To a query on the differences within the BJP over Modi's nomination, Raut distanced the Sena from the issue, saying: "It is their (BJP's) internal matter and we shall not comment on it."