Avijit Das Patnaik, 44, who lost his job at Singapore's DBS Bank over the incident, on August 14 posted the picture on the Facebook page of the Singapore Indians and Expats group, showing the Singapore flag on a T-shirt being ripped up to reveal the Indian flag underneath.
"Following investigations, the police, in consultation with the Attorney-General's Chambers, has administered a stern warning," Channel News Asia quoted the police as saying.
He was investigated under the Singapore Arms and Flag and National Anthem Rules which state that "no person shall treat the flag with disrespect", the police said.
The warning was given on October 3, about two months after a police report was made for his Facebook post, according to the report.
Patnaik has been a Singapore permanent resident for nine years. Some netizens commented that the post was offensive and insulting to Singapore. It was then taken down.
Patnaik, who lives here with his wife and two children, remains unemployed as the incident resulted in him losing the job at the DBS Bank following internal probe and disciplinary action.