Madonna suffered a mishap onstage this weekend ... which saw her go toppling backward in the middle of the show -- but thankfully, she seemed to be okay.

The singer was in the middle of performing Sunday night at Climate Pledge Arena in Seattle -- her latest stop on the "Celebration" tour -- and while she was belting out "Open Your Heart," she and one of her backup dancers attempted this choreographed move on a chair.

In video obtained by TMZ, you can see Madge sitting in the chair -- which her dancer tilts back ... only to lose control and send his boss straight to the floor. It stunned the crowd.

Unclear what exactly went wrong here ... but from the looks of it, the dude lost his grip as he was dragging her back down the stage plank, and it ended with Madonna on her back.

Madge was laying there for a quick sec, but the show went on ... with her dancer continuing to strut his stuff -- and Madonna herself eventually rising to her feet and keeping it going.

Funny enough ... something similar happened at yet another concert of hers recently -- only it wasn't the pop star in the chair, but rather ... Old Saint Nick, who took quite a tumble.

Chair routines onstage -- pretty tricky, and seem to end with folks laid out on their ass.