"I place my faith in and am grateful to Allah the almighty to announce that tomorrow, Thursday May 1, 2014, will see the enforcement of sharia law phase one, to be followed by the other phases," the absolute monarch said in a royal decree Wednesday.
The sharia penalties -- which will eventually include flogging, severing of limbs and death by stoning -- triggered condemnation on social media sites in the tiny sultanate earlier this year.
Confusion has swirled around implementation following the unexplained postponement of an expected April 22 start date that raised questions over whether the Muslim monarch was hesitating.
But the 67-year-old sultan -- one of the world's wealthiest men -- said the move was "a must" under Islam, dismissing "never-ending theories" that sharia punishments were cruel, in comments clearly aimed at detractors.
"Theory states that Allah's law is cruel and unfair but Allah himself has said that his law is indeed fair," he said.
The monarch's wealth -- estimated three years ago at $20 billion by Forbes magazine -- has become legendary with reports of a vast collection of luxury vehicles and huge, gold-bedecked palaces.
The monarchy was deeply embarrassed by a sensational family feud between Hassanal and his younger brother Jefri Bolkiah over the latter's alleged embezzlement of 15 billion dollars during his tenure as finance minister in the 1990s.
Court battles and exposes revealed salacious details of Jefri's un-Islamic jet-set lifestyle, including allegations of a high-priced harem of Western paramours and a luxury yacht.
Bruneians enjoy among the highest standards of living in Asia due to the country's energy wealth, with education, medicine and other social services heavily subsidised.
The sultan has for years discussed introducing the sharia penal code, as he warned of rising crime in his sleepy realm and pernicious outside influences such as the Internet. He announced the implementation plans in October.
Situated on Borneo island, which it shares with Malaysia and Indonesia, the small state already practised a relatively conservative form of Islam compared to its Muslim-majority neighbours, banning the sale of alcohol and heavily restricting other religions.
Muslim ethnic Malays, who make up about 70 percent of the population, are broadly supportive of the move by their revered father-figure.
But some Malays and non-Muslim citizens privately express unease. About 15 percent of Brunei's people are non-Muslim ethnic Chinese.
Earlier this year, many users of Brunei's active social media -- the only avenue for public criticism of authorities -- denounced the penal code as barbaric and out of step with the gentle Bruneian national character.
Theories abound as to why the sultan is pushing sharia, ranging from a monarch becoming more religious as he ages, to a desire to increase social controls in the face of a changing world.
He has called Islam a "firewall" against globalisation.
The initial phase beginning Thursday will introduce punishments including fines or jail terms for offences ranging from indecent behaviour, failure to attend Friday prayers, and out-of-wedlock pregnancies.
Officials have said a second phase covering crimes such as theft and robbery will start later this year and will include more stringent penalties such as severing of limbs and flogging.
Late next year, punishments such as death by stoning for offences including sodomy and adultery will be introduced.
Brunei currently has a dual-track legal system of civil courts along with sharia-compliant courts handling non-criminal issues such as marital and inheritance cases.
The UN's human rights office said earlier in April it was "deeply concerned" by the move, adding that penalties such as stoning are classified under international law as "torture or other cruel, inhuman or degrading treatment or punishment".
It said UN studies have shown that women are more likely to be sentenced to death by stoning, due to entrenched discrimination and stereotyping.
"It's a return to medieval punishment," said Phil Robertson, deputy Asia director for Human Rights Watch.
"It's a huge step back for human rights in Brunei and totally out of step with the 21st century."
Officials have sought to ease fears by saying such cases would require an extremely high burden of proof and judges would have wide discretion to avoid sharia punishments.