The bird was caught at a port in the financial capital Mumbai with "messages written in a Chinese-like script" on its wings, the Times of India newspaper reported.
"Initially, the police had registered a case of spying against the bird, but after completing their inquiry, they dropped the charge," the report added.
The unnamed bird was held under lock and key at a city hospital while police carried out an investigation.
That probe took an "astonishing eight months", the India office of the People for the Ethical Treatment of Animals (PETA) said in a Thursday statement.
PETA India said police had granted "formal permission for the hospital to release the pigeon" on Wednesday.
Local media reports said the bird fluttered away in good health.
The pigeon is the latest of several detained by Indian authorities on suspicion of espionage.
Border security officers took a pigeon into custody in 2016 after it was found carrying a threatening message to Prime Minister Narendra Modi close to India's border with arch-rival Pakistan.
Another pigeon was held under armed guard in 2010 after it was found in the same region with a ring around its foot and a Pakistani phone number and address stamped on its body in red ink.
Officials in that case directed that no one should be allowed to visit the pigeon, which police said may have been on a "special mission of spying".
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