Katharine McPhee got a big surprise after a performance in London Saturday ... a pretty awesome bachelorette party.
Katharine had just taken her bow on stage, when the cast and crew from her play, "Waitress," blindsided her with the celebration.
TMZ broke the story, 35-year-old Katharine got engaged to 69-year-old music legend David Foster last July. David was nowhere in sight -- as he shouldn't be since the obligatory fake firefighter/stripper showed off his stuff.
Katharine tried to accelerate things a bit, wearing a bridal veil.