This article presents a series voltage-combining Doherty power amplifier (PA) achieving high output power (Pout) and high power-back-off (PBO) efficiency for 28-GHz fifth-generation (5G) applications. We introduce a new transformer-based series output combiner design method to achieve a true-Doherty load modulation that uses a compact footprint. The output stages of the main PA and the auxiliary PA (aux. PA) both use a differential three-stacked FET topology for high output power without posing a reliability issue. The intermediate-node matching is achieved by using a shunt inductor for voltage waveform alignment and efficiency improvement. A modified differential quadrature hybrid is proposed to achieve the desired quadrature power splitting function without the need for an input balun. For the proof of concept, the proposed PA is implemented in a 22-nm CMOS fully depleted silicon-on-insulator (FD-SOI) technology with a core area of 0.2 mm&#x00B2. At 28 GHz, the measured saturated output power (Psat), 1-dB output compression point (OP1dB), and peak power-added efficiency (PAE) are 22.5 dBm, 21.1 dBm, and 28.5&#x0025, respectively. State-of-the-art International Technology Roadmap for Semiconductors (ITRS) figure-of-merit (FOM) and power density are achieved. The measured PAE at 6-dB PBO is 22.1&#x0025, which results in an efficiency enhancement ratio of 1.56/3.12 with respect to an ideal class-B and class-A PA. The proposed PA can support 2.4-Gb/s 64-quadratic amplitude modulation (64-QAM) and 0.8-Gb/s 256-QAM signal with a competitive average PAE. Excellent device reliability is validated by operating the implemented PA under 3-dB gain compression point for 12 h with less than 0.06-dB Pout variation. The proposed Doherty PA with a compact footprint is suitable for the integration purpose of future 5G multiple-input multiple-output (MIMO) and phased-array applications.