Several hundred people had gathered outside the hotel to cheer Modi as he arrived in a security motorcade from the airport.
Stepping out of his car, Modi began to reciprocate their greetings by mingling and being among them in an unusual gesture by a visiting VIP in a high-security city.
Modi's disregard for security was in contrast to New York City's security consciousness.
It has hunkered down this week as more than 140 prime ministers, presidents and royalty descended on the city for the UN General Assembly meeting.
Vast stretches of Manhattan around the UN building and elsewhere have been cordoned off and access severely limited.
Several federal and city security forces, some in battle gear, patrol key areas or staff checkpoints. Behind the UN building, Coast Guard boats have taken up positions on the East River, while police helicopters keep watch overhead.
Traffic is blocked off on the side streets along the New York Palace Hotel on Madison Avenue where Modi is staying. Visitors to the hotel have to pass through airport-style screening by federal agents to get inside.