We study the effect of introducing a market for green energy attributes on the market for the energy itself. In Europe, renewable energy producers receive Guarantees of Origin (GOs) that they can sell to consumers who wish to declare their electricity consumption as “green”. In a model of price competition, we show how the introduction of such a GO market can increase competition in the spot electricity market, leading to reduced electricity prices. In the current market design, the trade of GOs is not restricted by the physical transmission capacity in the spot electricity market. However, since the production capacity of GOs is still limited by the total dispatch of electricity, suppliers have incentives to compete more fiercely in the spot market. This pro-competitive effect disappears if the physical transmission capacity is also imposed on the GO market.