While previous research has analysed the links between drinking water access and health, and to some extent between health and agriculture, the direct impact of household water access on agricultural productivity has hardly been studied. We address this research gap, using survey data from Tanzania. Regression models show that water access constraints significantly decrease labour productivity for households that require more than the median time for water collection, doubling the time required for water collection reduces labour productivity by over 20%. For these water-constrained households we also identify a negative effect of water collection time on crop yield. Since we control for differences in irrigation, input use, crop type and other factors, we conclude that the productivity effects are mainly due to poorer health in water-constrained households. These results suggest that there are important linkages between drinking water access and agricultural growth which have often been overlooked in the past. Improving water access for rural households should receive higher policy priority not only from a health perspective but also from an agricultural growth perspective.