Smacking is now illegal in Scotland, making it the first part of the UK to bring in such a ban.
Children have the same protection from assault as adults after the change came into force on Saturday.
The Scottish Parliament approved the move last year, and Wales is expected to follow suit by 2022.
Smacking is still permitted in other parts of the UK if it is "reasonable punishment", according to the Children's Act 2004.
What is deemed "reasonable punishment" will depend on each case, but it cannot be used to justify serious physical punishment such as wounding, actual bodily harm and grievous bodily harm.
Children's minister Maree Todd said: "I'm very pleased that Scotland has become the first part of the UK to legislate to ensure that children, without exception, have the same protection from assault as adults.
"This outdated defence has no place in a modern Scotland. It can never be reasonable to strike a child."
Joanna Barrett, from NSPCC Scotland, said it was a "common-sense move" to scrap the justifiable assault defence.
She said: "This law sets out in clear terms that physical punishment should no longer be part of childhood in Scotland and it marks a momentous step in making it a country where children's rights are truly recognised, respected and fulfilled."
Green MSP John Finnie, who introduced the changes, said that during his campaign he met many people who thought smacking was already illegal.
Be Reasonable Scotland opposed the legislation and the campaign group warned it could mean parents being prosecuted for "even the mildest physical discipline".
A spokesman said: "In the years ahead, loving parents who have had no contact with the authorities previously and who present no risk to their children will face stressful intervention, blacklisting on police databases and even criminal records for smacking.
"The majority of Scots see this as an injustice, not a positive change."