With this, Congress's strength in the 130-member house has dwindled to 11.
Ravindra Phatak, who was once with Shiv Sena and known to be close to senior Congress minister Narayan Rane (who too was once with Sena), and six others today joined the saffron party in the presence of Shiv Sena president Uddhav Thackeray at the Thackeray residence `Matoshree' in Mumbai.
Local Sena MLA Eknath Shinde, MP Rajan Vichare and MLA Pratap Sarnaik too were present.
Shiv Sena's strength in the corporation now is 60.
Party sources said that Phatak may get ticket from Thane in the coming Assembly elections.
Reacting to the development, Thane Congress chief Bal Purnekar said he knew from the day Phatak crossed over to Congress that he would not remain loyal and go back to Sena.
Phatak's defection back to Sena would not affect Congress, said Purnekar.
Phatak told reporters that he was feeling "suffocated" in Congress and hence decided to quit.