Reaching over six metres, the so-called century plant is due to flower in approximately two weeks' time.
It matures slowly and dies after flowering but continues its lineage by producing offsets at the base of the stem throughout its life (as well as large quantities of seed), which are easily propagated.
The flower spikes can reach over 10m and sport bright yellow-green flowers that are pollinated by hummingbirds in the wild.
Kew director Professor Stephen Hopper said: "We are extremely excited here at Kew to catch a glimpse of the century plant at the peak of its life cycle. Visitors who come to Kew over the next month will get a rare opportunity to see a truly impressive botanical occurrence. This year at Kew, we are celebrating International Year of Biodiversity, and this specimen is a great example of the beauty, joy and economic use that we get from the plants we share our planet with."
Native to tropical America, the century plant was introduced to Padua Botanical Garden in Italy - the world's first botanical garden - in 1561 and is now widely cultivated throughout the world.
The species is now naturalised in the driest parts of southern Europe, and is often used for fencing in Mexico and Central America, as it is impermeable to both cattle and people once established due to its size and needle-sharp spines.
It was introduced to Spain in the 1940s for the production of sisal for rope, but subsided due to the arrival of nylon and synthetic ropes. In addition, the fermented juice of the agave plant is used to make the drink mescal.