Title: On Adversarial Bias and the Robustness of Fair Machine Learning. Abstract: Optimizing prediction accuracy can come at the expense of fairness. Towards minimizing discrimination against a group, fair machine learning algorithms strive to equalize the error of a model across different groups, through imposing fairness constraints on the learning algorithm. But, are decisions made by fair models trustworthy? How sensitive are fair models to changes in their training data? By giving equal importance to groups of different sizes and distributions in the training set, we show that fair models become more fragile to outliers. We study the trade-off between fairness and robustness, by analyzing the adversarial (worst-case) bias against group fairness in machine learning and by comparing it with the effect of similar adversarial manipulations on regular models. We show that the adversarial bias introduced in training data, via the sampling or labeling processes, can significantly reduce the test accuracy on fair models, compared with regular models. Our results demonstrate that adversarial bias can also worsen a model's fairness gap on test data, even though the model satisfies the fairness constraint on training data. We analyze the robustness of multiple fair machine learning algorithms that satisfy equalized odds (and equal opportunity) notion of fairness.