Sonawane, along with his supporters, joined Sena in presence of party president Uddhav Thackeray.
The MLA said he spoke to Raj Thackeray and Maharashtra Chief Minister Devendra Fadnavis before joining the Sena.
Assembly elections in Maharashtra are due in October this year.
Junnar assembly segment is part of Shirur Lok Sabha constituency, which is currently represented by Sena MP Shivajirao Adhalrao Patil.