Groups that continue to offer the “therapy” to anyone under the age of 18 could face up to a year in prison or a €30,000 (£26,268) fine.
Parents and guardians who force their children to attend such programmes can also be charged with violating their duty of care under the new law.
Proponents of gay conversation therapy claim to be able to “cure” and change the sexual orientations of LGBT+ people through psychotherapy. All major counselling and psychotherapy bodies in the UK, as well as the NHS, have condemned the discredited practice, according to LGBT+ charity Stonewall.
Germany’s health minister Jens Spahn, who is gay, said: “Young people are being forced into conversation therapies and so it is very important that they should find support in the existence of this law: a clear signal that the state does not want this to happen.
“Homosexuality is not an illness. Therefore the term ‘therapy’ is already misleading.”
However, the opposition Green Party – who were the first to call for the ban in 2013 – said the new law did not go far enough and the age limit should be raised to 26. The Left party, the fifth largest in the Bundestag, wants to raise it to 27.
Green legislators wrote: “Only minors are to be protected from this life-endangering charlatanry. At the very least, young people aged between 18 and 26 need comparable protection, as is shown by the experiences of coming out and many young people’s dependence on their families.”
Mr Spahn did not specify reasons for the age limit, but German law makes it easier to protect minors, whereas freedom of speech and conscience laws are seen as more problematic to implement for over-18s.
Conversion therapies are banned in much of South America, in Switzerland, and in many US states.
Advocates say the bans spare young people pain, suffering, harassment and humiliation over their sexuality.
According to the Magnus Hirschfield Foundation, a human rights organisation in Berlin, some 1,000 people are subjected to conversion therapy in Germany each year.
Additional reporting by agencies
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