The 30-year-old Selvaraj, a lathe operator hailing from Namakkal district, had developed a relationship with the woman from Theni through his habit of making calls to unknown numbers and was in touch with her over phone for the last 10 months.
She came to the city some five days back and stayed with him for four days in his house, police said.
As the woman insisted on marriage and threatened to reveal the relationship to his wife and family members, Selvaraj allegedly smothered her with a pillow on November 16, police said.
As bad smell emanated from the house, neighbours broke open the door and found the decomposed body two days back, they said. Police managed to trace Selvaraj from a hideout in Karur and arrested him, they said.