Intertidal seaweeds inhabit an inherently stressful environment with rapidly changing physical conditions with the turning tides. Many macroalgae are therefore very resistant to abiotic stress; however, the bases for this tolerance and the relative importance of different stressors are largely unknown. Here, the effects of stress on the transcriptome of the red seaweed Chondrus crispus were investigated using cDNA microarrays. The responses were studied after exposure to high light, high temperature, and hypo- and hyperosmotic conditions in the laboratory and compared with gene expression in nature at different stress loads: at high and low tide at solar noon, and during a cloudy and a sunny day, respectively. The study identifies key stress genes and marker genes for specific stressors. The data also provide an insight into the physiological effects of stress; for example, high light stress and high natural stress caused an increase in antioxidative proteins, suggesting an increased oxidative stress. Clustering analysis suggested that osmotic stress modulated the gene expression in nature under high-stress conditions and was thus the most significant natural stressor. The potential cross-talk between stress reactions and methyl jasmonate-induced responses was also investigated and is tentatively suggested to be mediated by reactive oxygen species.