Ustin the tiger was spotted some six or seven kilometers away from the border on the Bolshoi Ussuriysky Island that is located at the confluence of the Amur and Ussuri rivers and is divided between China and Russia.
Sergei Aramilev, the head of the local branch of the Amur Tiger Centre, said that Ustin was unlikely to stay at the Chinese part of the island, which lacks food for the feline.
Meanwhile, the second tiger, Kuzya, who was located in Taipinggou nature reserve in northeastern China in early October, "has gone too far... and will most like stay there," Aramilev said.
He said both tigers could have left Russia in search for new habitats.
Kuzya and two other tigers were released into the wild by Russian President Vladimir Putin in May.
The Siberian tiger, also known as Amur tiger, is on the Red List of Threatened Species. In 2010, Russia launched a national strategy to protect the Siberian tiger, the largest of the five tiger species. According to World Wildlife Fund, some 450 Siberian tigers are left on the planet.