Smacking children is now illegal in Wales, in a move campaigners have called a "historic moment for children and their rights".
First Minister Mark Drakeford introduced the new law, which makes all forms of physical punishment prohibited, under the Children (Wales) Act.
Children will now have the same protection from assault as adults - and the law will also be applicable to anyone visiting Wales.
Wales joins 60 other countries with such bans.
Drakeford said of the legislation: “No more grey areas. That is all in the past. There is no place for physical punishment in a modern Wales.”
Scotland introduced its own ban in November 2020.
Previously in Wales, and as is still the case in England and Northern Ireland, smacking a child was unlawful, but such an assault was allowed as long as it constituted "reasonable punishment".
Whether the defence was accepted depended on the circumstances of each case, taking into consideration factors such as the age of the child and the nature of the contact, including whether it left a red mark or was carried out with a fist or implement such as a cane or belt.
Deputy Minister for Social Services Julie Morgan called the change in law a "historic moment for children and their rights in Wales".
It comes after a study found that smacking children does not improve their behaviour and may actually increase behavioural difficulties.
Researchers from University College London (UCL) analysed 20 years of research on physical punishment by looking at 69 global studies.
They followed children over time and assessed data on smacking, finding that approximately 250 million children around the world are subject to physical punishment.
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The study came to the conclusion that smacking can in fact make things worse.
Lead author Dr Anja Heilmann, from UCL's Department of Epidemiology, said: "Physical punishment is ineffective and harmful, and has no benefits for children and their families. This could not be clearer from the evidence we present.
"We see a definitive link between physical punishment and behavioural problems such as aggression and antisocial behaviour.
"Even more worrying are findings that children who are the recipients of physical punishment are at increased risk of being subjected to more severe levels of violence."
The findings, published in medical journal The Lancet, suggest the link between physical punishment and increased behaviour problems is causal.
However, there were no signs that smacking resulted in a reduction in bad behaviour.
The study added that smacking also showed no improvement in children's attention, cognitive abilities or social competence..