A solution-processed polymer tandem cell fabricated by stacking two single cells in series is demonstrated. The two bulk-heterojunction subcells have complementary absorption maxima at max 850 nm and max 550 nm, respectively. A composite middle electrode is applied that serves both as a charge-recombination center and as a protecting layer for the first cell during spin-coating of the second cell. The subcells are electronically coupled in series, which leads to a high open-circuit voltage of 1.4 V, equal to the sum of each subcell. The layer thickness of the first (bottom) cell is tuned to maximize the optical absorption of the second (top) cell. The performance of the tandem cell is presently limited by the relatively low photocurrent generation in the small-bandgap polymer of the top cell. The combination of our tandem architecture with more efficient small-bandgap materials will enable the realization of highly efficient organic solar cells in the near future.