Ranariddh, now living in Malaysia’s capital Kuala Lumpur, had been sentenced in absentia to 18 months in jail in March last year over a 3.6 million dollar illegal property sale.
The court ruled he improperly sold his former political party headquarters and used proceeds from the sale to purchase another property in his own name.
Hours after Hun Sen was formally re-elected the country’s prime minister for another five-year-term, Sihamoni issued a royal pardon for his half-brother Ranariddh upon the request of the premier.
Sihamoni ordered Hun Sen to “successfully execute the royal decree.”
In a letter to the king, read out on television, Ranariddh thanked Sihamoni for giving him “full freedom in order to join in the development of the nation.”
Ranariddh also thanked Hun Sen for helping to arrange the royal pardon for him in another letter, and told the premier he would return to Cambodia on Sunday.
Ranariddh faced jail once before in 1998 but was spared by a royal pardon from his father, former king Norodom Sihanouk.
He had been sentenced to 35 years in prison for allegedly plotting a coup with the Khmer Rouge a year earlier while he was co-prime minister with Hun Sen.