Titan Arum produced a bud several weeks ago, which has recently been growing at several centimetres a day.
But up until Friday morning it had not been clear if it would produce a leaf, or a flower.
It will be the first time the so-called "corpse flower" has ever blossomed in Scotland.
It gets its name from the stench of decaying flesh it produces, to attract insects to pollinate it.
Staff at the Garden said "it's going to be incredible" when the full bloom shows.
But that could take at least a week.
And the full bloom, when it does appear, may only last for 48 hours.
The corm was gifted to the Botanic Garden in 2003.
It produced a similar growth spurt four years ago but that turned out to be a giant leaf structure which reached to the roof of the glass house.
Experts at the Royal Botanic Garden say one of the world's biggest - and smelliest - flowers is about to bloom in Edinburgh.