In-context imitation learning is the capability to perform novel tasks when prompted with task demonstration examples. In-Context Robot Transformer (ICRT) is a causal transformer that performs autoregressive prediction on sensorimotor trajectories, which include images, proprioceptive states, and actions. This approach supports flexible and training-free execution of new tasks at test time. Experiments with a Franka Emika robot demonstrate that ICRT can adapt to new environment configurations that differ from both the prompt and the training data. In a multi-task environment setup, ICRT significantly outperforms current state-of-the-art robot foundation models on generalization to unseen tasks. Code, data, and appendix are available on https://icrt.dev.