Zac Hanson -- MAN SPITS IN DRUMMER'S FACE ... Caught On Video

Zac Hanson -- the drummer from Hanson -- was the victim in a disgusting spit attack on his tour bus in Seattle last night ... and the gross moment was all caught on video.

Here's what we know -- the spitter somehow got onto Hanson's tour bus ... and as Zac was trying to boot him from the bus, the guy unleashed a big nasty loogie that hits Zac right in the head.

You can see the spit dripping off Zac's hair ... it's really disgusting ... but Zac simply brushed off the incident, and smiled as if it was no big deal.

As for the douche bag ... he simply turns and walks away while cursing out some of the people surrounding the bus.

After the incident, Zac tweeted ... "Gonna be a good show tonight, I cleaned off all the spit and I am feeling ready to go! -Z"