Punishment of defectors is believed to be a key factor in sustaining cooperation among unrelated individuals. However, several studies have shown that humans also frequently punish those that act cooperatively, a phenomenon termed anti-social punishment. While subsequent work has suggested reasons why anti-social punishment may be individually optimal, it is universally considered to be detrimental to cooperation and, thus, indeed, “anti-social.” This study contradicts this view, showing that punishment of cooperators can be a positive factor in sustaining cooperation rates when used by conditional cooperators against those who cooperate unconditionally. This suggests that judging whether a punitive act is beneficial or detrimental to cooperation is more complex than previously thought.