After fierce debate the country relaxed laws surrounding euthanasia two years ago to allow children to be helped to die.
It's not been revealed by authorities whether the terminally ill 17-year-old was male or female or what condition they were suffering with. It is understood the child was in constant pain.
Euthanasia of children over 12 is also legal in Holland, where it's thought dozens have been helped to die.
Belgian law states a child can be helped to die if:
They are in the final stages of a terminal illness
They request to be helped to die several times
They have their parent's permission
Alistair Thompson is from Care Not Killing, a campaign group of both Christians and non-Christians.
Speaking on Premier's News Hour, he said: "The vast majority will be completely revolted by the fact that children who're not allowed to vote, who're not allowed to drive, who're not allowed to join the armed forces, have the right to request that a doctor kills them.
"It's a deeply depressing and it shows what happens when you start to introduce a law for mentally competent adults and has slowly been extended to children.
"We're crossing a very very dangerous rubicon... We're on a very slippery slope to starting to ration treatment because it's too expensive.
"Please revise the [assisted suicide] law and get rid of it."
Belgian senator Jean-Jacques De Gucht told the Daily Mail: "It gives me some comfort to know that now there is a choice out there for children in the final terminal stages."
Listen to Premier's Hannah Tooley speaking to Alistair Thompson on the News Hour: