Line a 9x11-inch baking pan with parchment paper, leaving a 2-inch overhang on two sides.

Place peanut butter, protein powder, and honey in a large bowl; mix with a wooden spoon to combine. Add goji berries, flax seeds, and oats, stirring well to combine. Spread mixture evenly in baking pan.

Place baking pan in the refrigerator and chill until mixture is set, about 1 hour. Lift out of the baking pan; cut into 3-inch squares.