The songstress, 30, showed off her svelte body after giving birth to daughter Blue Ivy just five months ago -- her first child with husband Jay-Z.
"Y'all have no idea how hard I worked!" the 30-year-old told fans at Saturday night's performance. "I had to lose 60 pounds. They had me on that treadmill. I ate lettuce!"
Now that she is almost through with her four-day Memorial Day concert, she plans to celebrate, exclaiming, "Tonight I'm gonna get chocolate wasted!"
First Lady Michelle Obama and daughters Malia, 13, and Sasha, 10, were in attendance Saturday, dancing up a storm to Beyonce songs "Crazy in Love" and "Single Ladies."