In this letter, the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> -factor is estimated in a typical office scenario based on wideband channel measurements carried out at millimeter-wave (mmWave) frequencies, covering the 25-40 GHz spectrum, in both line-of-sight (LOS) and obstructed-LOS (OLOS) propagation conditions. The <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> -factor is estimated from the method of moments, applied directly over the frequency samples of the measured complex channel transfer function (CTF), and from the power delay profile (PDP) based method. Although both methods provide similar results, the method of moments is more appropriate from a practical point of view, especially in OLOS and non-LOS (NLOS) conditions where the correct identification of dominant components can be difficult. The results are particularized to the potential 26, 28, 33 and 38 GHz frequency bands for the design and deployment of the future wireless networks at mmWave frequencies. The mean value of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> -factor ranges from -1.87 to 1.27 dB for LOS condition, and from -3.79 to -2.31 dB for OLOS condition.