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This article describes the design and measured performance of a linear efficient Ka -band InP DHBT PA with an IP3/ $\text{P}_{\mathrm {dc}}$ linearity figure of merit (LFOM) of 19.8:1 at 48 GHz and 14.1:1 at 40 GHz. The MMIC PA is based on an InP DHBT technology with peak ${f}_{AT}$ and ${f}_{\mathrm{ max}}$ of $>$ 300 and $>$ 600 GHz, respectively, and achieves among the best LFOM at the Ka -band above 40 GHz from an MMIC-matched amplifier. The amplifier is a single-stage four-way combined design that was optimized for a combination of power and PAE at < 2-dB compression. The linear ${P}_{\mathrm{ 1\,dB}}$ is 21.2 dBm with a PAE of 28.3% and a corresponding IP3 of 36.1 dBm. A remarkable IP3- ${P}_{\mathrm {1\,dB}}\,\,=$ 14.9 dB is obtained which is significantly higher than the 10 dB rule of thumb for semiconductors. At 48 GHz, a peak IP3 of 36.6 dBm and LFOM of 19.8:1 is achieved which is twice better than the low microwave frequency rule-of-thumb of 10:1 for LFOM. Under a 200-MHz QPSK modulation, linear PAEs of 17.4%–19.5% for 5% EVM across a 40–44-GHz band were achieved. This work demonstrates among the highest IP3/ $\text{P}_{\mathrm {dc}}$ LFOM and EVM linear PAE reported for an MMIC above 40 GHz.