A quasi-optical interferometer operating at the frequency f = 274.2 GHz has been employed for measuring electron densities of pulsed plasmas in a wide range. Line-integrated densities down to nearly 10¹⁴ m⁻² can be measured with temporal and spatial resolutions of 1 µs and less than 10 mm, respectively. A wavelength of about 1 mm requires a quasi-optical set-up making use of grid and dielectric beam splitters as well as elliptic mirrors for focussing the probing and reference beams. Due to the Michelson-like set-up, the probing beam traverses the plasma twice thus doubling the sensitivity as compared with the Mach–Zehnder configuration. The large difference between the probing and reference paths enables one to shift the phase by varying the frequency in a narrow band. When the detector output signal of the interferometer is recorded thereby shifting the phase stepwise, one obtains a cosine function for each sample time from which the phase shift by the plasma and, thus, the electron density, is deduced. The performance of the interferometer is demonstrated on a pulsed high-density helicon discharge as well as the large-volume plasma in a diffusion chamber connected with the helicon source.