Some 11.90 million subscribers were added in April compared with 15.
64 million during the previous month, figures posted on the Telecom Regulatory of India (TRAI) website said.
The rise took India's total wireless subscriber base to 403.
66 million, TRAI said.
The slowdown in subscriber growth came after cellular operators withdrew special deals on offer during the final months of the fiscal year to March when the firms sought to boost revenues to help their annual accounts.
But India remains the world's fastest-growing mobile market and analysts say the government's target of 500 million mobile phone users could be reached ahead of schedule.
The total telecom subscriber base made up of wireless and landline customers stood at 441.47 million at the end of April compared with 429.72 million in March, TRAI added.
Total penetration stands at close to 38 telephones for every 100 people, TRAI said. India has nearly 1.2 billion people.
India has said it will stage its much delayed