Abstract We present electron densities n _e in the interstellar medium (ISM) of star-forming galaxies at z = 4–9 observed by the JWST/NIRSpec GLASS, Early Release Observations, and CEERS programs. We carefully evaluate the line-spread functions of the NIRSpec instrument as a function of wavelength with the calibration data of a planetary nebula taken on board, and obtain secure [O ii] λ λ3726, 3729 doublet fluxes for 14 galaxies at z = 4.02–8.68 falling on the star formation main sequence with the NIRSpec high- and medium-resolution spectra. We thus derive the electron densities of singly ionized oxygen nebulae with the standard n _e indicator of the [O ii] doublet, and find that the electron densities of the z = 4–9 galaxies are n _e ≳ 300 cm⁻³ significantly higher than those of low-z galaxies at a given stellar mass, star formation rate (SFR), and specific SFR. Interestingly, the typical electron densities of the singly ionized nebulae increase from z = 0 to z = 1−3 and z = 4–9, which is approximated by the evolutionary relation of n _e ∝ (1 + z) ^p with p ∼ 1–2. Although it is not obvious that the ISM property of n _e is influenced by global galaxy properties, these results may suggest that the nebula densities of high-z galaxies are generally high due to the compact morphologies of high-z galaxies evolving by r e ∝ ∼ ( 1 + z ) − 1 (r _vir ∝ (1 + z)⁻¹) for a given stellar (halo) mass whose inverse square corresponds to the p ∼ 2 evolutionary relation. The p ∼ 1−2 evolutionary relation can be explained by a combination of the compact morphology and the reduction of n _e due to the high electron temperature of high-z metal-poor nebulae.