If the bill is signed into law, Portugal will become the fourth country in Europe to legalize euthanasia, alongside the Netherlands, Belgium, and Luxembourg. Around 81% of Portugal's 10 million population are baptized Catholics.
The bishops said that accepting the law would "convey the erroneous idea that life marked by illness and suffering no longer deserves protection and becomes a burden to oneself, to those around them, to health services and to society as a whole."
They added: "The response to illness and suffering should rather be the protection of life, especially when it is more fragile, by all means, and especially by access to palliative care, which the majority of the Portuguese population is still deprived of."
Portugal, while still a Catholic-majority country, has legalized same-sex marriage and abortion in the past several decades.
The Socialist Party, one of the left-of-center parties leading the charge to push the euthanasia legislation in Portugal, also led proposals to permit same-sex marriages and abortion in Portugal, the AP reports.
Portugal's parliament failed to pass several proposals to legalize euthanasia and assisted suicide in mid-2018.
When lawmakers began debating five pieces of legislation in February 2020 to decriminalize euthanasia and assisted suicide, doctors in the country joined with the Catholic Church in opposing the potential change.
The Portuguese Doctors' Association says the legislation violates key principles of the medical profession.
"Doctors learn to treat patients and save lives. They are not prepared to take part in procedures leading to death," PDA president Miguel Guimaraes said after meeting with President Rabelo de Sousa.
Pro-life groups protested the euthanasia bills in the weeks leading up to the vote in Lisbon, where they held signs saying, "We demand palliative care for ALL" and "Euthanasia is a recipe for elder abuse."