The blaze began around 4:25 p.m. and ripped through the building at 770 Rutland Road.
No one was hurt, but residents of all 64 apartment units are looking for somewhere to sleep tonight.
"While our crews were enroute, they noted a heavy black column of smoke coming from the building," said Kelowna's deputy fire chief, Lou Wilde.
"It's extensive, the fourth floor roof is off. There's heavy damage on the fourth floor. Third floor damage is significant."
Wilde said they are continuing to investigate the cause of the fire.