President Marcelo Rebelo de Sousa said the wording of the second draft still needs "clarification".
Portugal’s president has refused to sign a second draft bill legalising euthanasia and physician-assisted suicide.
President Marcelo Rebelo de Sousa said the wording of the proposed law was too imprecise and vetoed the law.
President Rebelo de Sousa - a staunch Catholic - had previously expressed reservations about the first draft bill to legalise euthanasia.
The legislation has now been effectively shelved until a new Portuguese parliament is chosen in January's election.
Portugal has for years been debating euthanasia when a doctor directly administers fatal drugs to a patient.
Left-wing MPs have also called on parliament to allow medically assisted suicide, where patients administer lethal drugs themselves under medical supervision.
Earlier this year, de Sousa referred the first reading of legislation on the subject to the country's constitutional court. The court rejected the proposed bill, stating the law lacked "indispensable rigour".
After a second vote, MPs approved a rewritten version of the law, but de Sousa has now sent the law back to Portugal's parliament.
The President argued that the law needed to better clarify "contradictions" to what justifies medically assisted suicide in cases of "fatal", "incurable" or "serious" illnesses.
Without the need for patients to be terminally ill changes Portugal's "values of life and free self-determination," he said in a statement.
Rebelo de Sousa has remained popular for more than five years as head of the Portuguese state.
The mostly Catholic country previously passed laws allowing abortion in 2007 and same-sex marriage in 2010.
In Europe, only Spain and the Benelux countries - Belgium, the Netherlands, and Luxembourg - have legalised euthanasia.