The singer-songwriter performed at the Palestine Will Live Forever Festival in Seattle Saturday night ... benefiting a charity known as the United Nations Relief and Works Agency for Palestinian refugees.

Watch the clip ... Macklemore's walking around the stage -- telling people to "straight up" say the phrase 'cause he ain't going to stop them.

He then says, "f*** America," -- and, his audience bursts into wild cheering before the clip ends.

Macklemore lambasting the U.S.A. has become pretty common in the last few months as the war between Israel and Hamas has continued to intensify.

In the spring, Macklemore released his song "Hind's Hall" -- named after the building at Columbia University taken over by pro-Palestine students -- which calls out Joe Biden and other politicians for backing Israel in the conflict.

Some Israel supporters -- like singer Matisyahu -- have blasted Macklemore for his song and views ... calling the track straight-up anti-Semitic.

Macklemore's sticking by Palestine though ... seemingly happy to slam his own country in the process.