Poachers broke into the cage of six-year-old female Royal Bengal tiger Oni in Itanagar zoo on Monday night and hacked it to pieces.
The incident happened when guards had gone out for dinner.
Arunachal Pradesh government has sought the assistance of the Wildlife Crime Control Bureau to probe the incident.
"We have written to (them)," P Ringu, deputy chief conservator of forest (wildlife) said.
The zoo houses many endangered species like clouded leopard, hoolock gibbon, Slow Loris, capped Langur and Royal Bengal Tiger.