One person injured in the blaze was taken to hospital, while a second guest with minor injuries is still being evaluated at the 2,995-room hotel.
The pair both suffered smoke inhalation, officials said.
Firefighters extinguished the blaze around 30 minutes after arriving at the scene, Fire Chief Greg Cassell told KSNV. An investigation is ongoing.
On Saturday afternoon,the Clark County fire department announced via Twitter that firefighters had been dispatched to a blaze at The Cosmopolitan.
In total, 110 firefighters and 35 vehicles, including crews from Las Vegas Fire and Rescue, responded to the fire, which started at 12.15pm local time.
'@ClarkCountyFD fighting a #fire at 3708 Las Vegas Blvd So. 2nd alarm called. #CCFD #vegas,' Clark County Fire Department tweeted.
It later revealed that trees and cabanas were burning on the resort casino and hotel's pool deck.
'Update to 3708 LVBS: Trees and cabanas burned on pool deck. Very little extension to building. One patient transported at this time,' it tweeted.
By the time crews had extinguished the blaze, it had already destroyed the Bamboo Pool, which is located on the hotel's 14th floor.
- Daily Mail