Yashini, hailing from the city, and 21 other transgenders received the appointment orders from City Police Commisioner Smith Saran after undergoing medical check-up.
The candidates will undergo SI training in Chennai, police said.
Yashini had moved the Madras High Court last year after her application for the post of SI was rejected by the Tamil Nadu Uniformed Services Recruitment Board (TNUSRB).
The court in November last had directed the TNUSRB to appoint her as SI as she is entitled to get the job and also asked it to include transgenders as third category.
Yashini told PTI today that her ambition was to become an IPS officer.
She said she will help transgenders like her get education and employment. "I will take action against those involved in child abuse and sexual harassment," she said.