The Constitutional Council's ruling will see France join the Netherlands, Belgium, Switzerland and Canada, which have already legalised the practice.
France's President Emmanuel Macron signed assisted dying legislation into law on Wednesday, after it was upheld by the Constitutional Council last week.
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Macron's office hailed the council's decision on Friday which was seen as one of the major social reforms of his second term.
The ruling "completes an exemplary democratic debate," the Élysée Palace said, adding that the legislation "provides an essential safeguard for our fellow citizens."
The legislation establishing a right to assisted dying for some adults suffering from an incurable condition was passed by parliament in July after years of debate.
The Constitutional Council's ruling will see France join the Netherlands, Belgium, Switzerland and Canada, which have already legalised the practice.
The decision is a victory for Macron, who promised the law after his re-election in 2022, marking one of France's biggest social reforms since same-sex marriage was legalised in 2012.
At the same time the constitutional authority found that certain provisions of the legislation required clarification, including the so-called conscience clause, which allows healthcare professionals to refuse to take part in the procedure.
The clause must also apply to private healthcare facilities where assisted dying is "manifestly contrary" to their mission, provided they are not the only establishments able to meet "local needs", the constitutional authority said, meeting a key demand from Catholic healthcare institutions.
To be eligible for assisted dying, a patient must be of legal age and a French national or a long-term resident.
They must suffer from pain that is either unresponsive to treatment or, in the patient's view, unbearable if they have chosen not to receive or to stop receiving treatment.
A physician is responsible for verifying the patient's eligibility before a panel assesses whether the criteria have been met.
The doctor ultimately makes the decision alone, and the patient may withdraw consent at any time.
The patient will administer a lethal substance themselves, with exceptions for those who are physically unable to do so and can be helped by a healthcare worker.