The singer-songwriter hopped onstage with the band for the fourth and final Melbourne date on their "Music of the Spheres" tour ... and, he should've kept his eyes on the ground 'cause he stepped in the wrong spot and almost took a nasty tumble.

Watch the video ... Martin's reading out signs he sees in the crowd in between songs when he steps backward and falls -- thankfully, into the waiting arms of a crewmember -- who stops him from smacking into the harsh floor.

The crowd reacts with a bunch of gasps ... but, Martin bounces up pretty quick -- jokingly telling fans that wasn't planned and thanking the stagehand for making the grab.

As always, the show must go on ... and CM gets right back into the performance -- so, seems he didn't sustain any sort of injury.

Martin isn't the first star to fall through a trapdoor ... just last month, Olivia Rodrigo did the same thing in Melbourne -- albeit, at a different arena. She was also uninjured and ended up performing her full show after.

Coldplay's finished in Melbourne ... but, they've got performances in Sydney and Auckland, New Zealand coming up this week.