Hailakandi District and Sessions Court Judge Debasish Bhattacharjee convicted Jashimuddin Borbhuiyan on Monday in the rape and murder case of the 13-year-old tribal girl.
The quantum of punishment will be pronounced on October 4.
The girl, who was a resident of Betcherra village, bordering Mizoram, was raped and brutally murdered by Borbhuiyan on March 14.
The incident had stirred an outrage among the locals who had taken out rallies and protest marches demanding stringent punishment for the perpetrator.
Congratulating Superintendent of Police Mohneesh Mishra and his team, Hailakandi Deputy Commissioner, Adil Khan said, "The SP and his team delivered what we had promised to the people of Hailakandi six months ago.
"This will reaffirm the faith of the common man in our administration, police, judiciary and the entire criminal justice system".