Ishida tried to trade punches and gamely stood his ground in the middle of the ring, but that made him only a bigger target and Golovkin sent him crashing down with a stunning overhand right that caught him on the cheek.
Ishida's head hit the bottom rope and he lay motionless for about a minute before getting back on his feet.
The 30-year-old Golovkin improved to 26-0, including 23 KOs, while the 37-year-old Ishida was stopped for the first time.