Amar Nath was arrested on Thursday night from his house in the slums of Jaitpur.
Amar Nath, who is originally from Maharashtra, lured the 11-year-old boy on February 28 while he was playing outside his school. He took the boy to an isolated place near a canal in Jaitpur and tried to sexually assault him.
As the boy resisted and threatened to reveal his crime, Amar Nath slit his throat and threw the body into the canal, Deputy Commissioner of Police Chinmoy Biswal said.
The body was found floating in the canal on March 3.
"During investigation, the boy's classmate said he had last seen him with a man wearing an earring, who was later identified as Amar Nath," the officer said.