<p>Extracellular osmotic pressure is a key modulator of intracellular biophysical properties and cellular functions. However, its impact on the cell nucleus remains elusive, largely due to the challenges in real-time measurement of local environmental properties and reaction kinetics at specific loci within the nucleus. Here, we employ the dCas9-SunTag system to investigate the biophysical response at target DNA loci to osmotic pressure alterations. We reveal that variations in extracellular osmotic pressure modulate the efficiency of dCas9-SunTag-mediated fluorescent labelling rapidly and reversibly, with hypoosmotic condition increasing and hyperosmotic condition decreasing the number and fluorescence intensity of foci for telomeres and genes. Strikingly, osmotic pressure also regulates gene transcription with the dCas9-SunTag system, mirroring its effects on fluorescent labelling, as evidenced by changes in mRNA burst frequency. The underlying mechanism is that osmotic pressure shifts the binding-unbinding equilibrium of specific proteins to dCas9-SunTag complex by altering intranuclear crowding. These findings not only highlight the role of mechanical cues in modulating DNA-related processes within the nucleus, but also establish the dCas9-SunTag system as a sensitive probe for intranuclear crowding in response to extracellular cues, notably osmotic pressure.</p>