Young people aged 16 and 17 are now legally banned from marrying in England and Wales.
Under the Marriage and Civil Partnership (Minimum Age) Act, it is now a crime to exploit vulnerable children by arranging for them to marry, or enter a civil partnership, under any circumstances.
That will be the case whether force is used or not.
"Traditional" ceremonies that are legally non-binding, but which are still viewed as marriages by the parties and their families, will be included.
Anyone found guilty of arranging child marriages faces a sentence of up to seven years in prison.
It is a "huge leap forward" in tackling "this usually hidden abuse", said Natasha Rattu, director of the Karma Nirvana charity, which is a member of the Girls Not Brides Coalition.
Ms Rattu said she hoped the new law would "help to increase identification and reporting, affording greater protection to children at risk".
Cases of child marriage
Justice Secretary Dominic Raab said perpetrators would face the "full force of the law".
In 2021, the government's forced marriage unit provided advice or support in 118 cases involving victims aged under 18.
Forced marriage is more likely to affect girls than boys, the Ministry of Justice said.
Figures for England and Wales in 2018 show 28 boys married under the age of 18, compared with 119 girls.
"Last year, the national Honour Based Abuse helpline supported 64 cases of child marriage, representing only a small picture of a much bigger problem," Ms Rattu said.
"This law will better protect vulnerable young people, by cracking down on forced marriage in our society," Mr Raab said.
"Those who act to manipulate children into marrying under-age will now rightly face the full force of the law."
Conservative MP Pauline Latham, who introduced the bill in parliament in 2021, said Monday is a "landmark day for the campaigners who have worked relentlessly for over five years to ban child marriage in this country".
She added: "Child marriage destroys lives and through this legislation we will protect millions of boys and girls over the coming years from this scourge."
Safeguarding minister Sarah Dines said the government is working to ensure that training and guidance is given to help police, social workers and other professionals "support and safeguard victims".