Stress response pathways rely on coordinated gene expression regulation. In the endoplasmic reticulum, the unfolded protein response plays an important role in maintaining the homeostasis of protein folding and processing. A chemigenetic reporter assay is developed that allows monitoring of BiP/GRP78 expression, a key regulator of the unfolded protein response. The system is based on the coexpression of the self‐labeling protein HaloTag via an internal ribosome entry site. HaloTag allows for flexibility in labeling color as well as labeling timepoint. A two‐color timestamping strategy is designed that provides improved readout sensitivity. The reporter system can be used in live‐cell imaging as well as flow cytometry. It is applied in a high‐content screening experiment in which a new promising activator of BiP/GRP78 expression was identified.