Thaksin travelled to Cambodia on November 10 for a five day trip to hold talks with Cambodian economists.
Siwarak, 31, worked for the Cambodia Air Traffic Service – which manages flights in the country.
He was arrested November 12 and charged with stealing information that could impact national security.
As well as the prison sentence, Siwarak was ordered to pay a $2,500 fine.
Judge Ke Sakhan, said: “Thaksin is Cambodia’s adviser, so the government of Cambodia has an obligation to protect his life.
“If anything happens to him, we would be blamed and that could lead to rocky relations with Thailand.”
Siwarak denied leaking the information and said he obtained the flight details because he wanted to know when his former prime minister would arrive.
Siwarak’s lawyer said he did not know yet whether his client would appeal the ruling.
Thaksin went into self-imposed exile last year before a Thai court found him guilty of violating a conflict of interest law and sentenced him to two years in prison.
He had served as prime minister from 2001 to 2006, when he was ousted in a military coup after facing corruption charges and being accused of disrespecting the monarchy.
Tuesday’s ruling is likely to heighten diplomatic tensions that reached boiling point when Cambodia refused to extradite Thaksin or recognise his conviction on the grounds that he was a victim of a vendetta by his political rivals.