Alexei Slabinskiy has been missing since March 13.
His sister Tatlana Novoselova said a 250,000 baht reward would be given to anyone that could provide information leading to the arrest of each of the two men wanted in connection with his disappearance - a Ukrainian national Andre Smirnov and a Russian, Alexandr Novichkov.
The two men are already wanted on arrest warrants for kidnapping and detaining of Yana Strizheus, Mr Slabinskiy's girlfriend, and demanding a ransom in exchange for her release. The warrants were issued on March 19.
The Russian suspect will also be charged with attempted rape of Ms Strizheus, who has been in Thailand since Aug 2, 2013 on a student visa.
Mr Slabinskiy's sister said another 100,000 baht reward would be given to anyone that can help find her 39-year-old brother, who is a property developer.
Mis Novoselova has come to Thailand with her husband to try and find her younger brother.
The reward money has been raised by Mr Slabinskiy’s friends, she said.
A police investigation found that shortly before the couple went missing they made calls to friends and relatives in Russia claiming they were involved in a road accident in Phuket and needed US$100,000 (3.2 million baht). No official reports of such an accident were made.
After the calls home, their mobile phones were not used again. Concerned relatives in Russia contacted the Russian embassy in Bangkok, which then alerted the tourist police in Phuket.