A spiral-coupled-line-based Ka -band (26.5–40 GHz) CMOS Wilkinson power divider with a shunt capacitor (C) at the input and a shunt RC (or C) between the outputs is reported. The spiral coupled line, i.e., two noninverting spiral transmission lines (TLs) with magnetic coupling, has a positive mutual inductance. This leads to a significant TL-length reduction (from about $\lambda $ /7 to $\lambda $ /12) for a specific TL series inductance. At 33 GHz, the power divider achieves prominent $S_{11}$ of −21.5, $S_{22}$ of −22.1, $S_{33}$ of −22.1, $S_{32}$ of −26.1, $S_{21}$ of −4.063, and $S_{31}$ of −4.052 dB. The corresponding amplitude imbalance (AI) and phase difference (PD) are −0.011 dB and −0.336°, respectively. Moreover, $S_{11}$ is smaller than −10 dB for 0–45.3 GHz, corresponding to 137.3% fractional bandwidth (FBW). $S_{32}$ is smaller than −10 dB for 24.1–40.2 GHz, covering the Ka -band and corresponding to 48.8% FBW. The power divider occupies a normalized area of $1.2\times 10^{-4}\lambda _{0}^{2}$ , one of the smallest normalized chip areas ever reported for millimeter-wave power dividers.