This paper proposes a probabilistic U-Net model for image segmentation that models segmentation uncertainty. The model consists of a prior network that predicts an initial segmentation map and a posterior network that refines the prior map based on the input image. The loss function maximizes the posterior probability of the ground truth segmentation while minimizing the divergence between the prior and posterior predictions, allowing the model to capture ambiguous regions. Experiments on medical images show the probabilistic U-Net produces better segmentations and uncertainty estimates than deterministic segmentation models.