Terahertz (1-10 THz) frequencies are among the most underdeveloped electromagnetic spectra, even though their application potentials are great in imaging, sensing, and communications1. This underdevelopment is primarily due to the lack of compact and powerful THz sources. Here, we report a compact and portable high power THz source based on THz quantum cascade lasers (THz QCLs2) with a record maximum operating temperature of 250 K, in a thermoelectric cooler (TEC) system. This record, which is substantially higher than the existing records, along with the novel design strategy to achieve it, is a significant development in the field of THz QCL. The compact THz QCL system developed in this work is all solid-state, robust and portable, and it can be operated anywhere with an electric outlet. The system generates sufficient power to perform real-time THz imaging as well as fast spectral measurements using a room-temperature camera and detector. As such, the overall imaging and sensing systems are portable. Such systems will have a qualitative impact on broad-range applications in biomedical imaging, sensing, and security. The invention of THz QCLs2 held great promise to bridge the so-called “THz gap” between semiconductor electronic and photonic devices. As fundamental oscillators, THz QCLs can generate much greater power levels (>1 W)3, and achieve much greater cw power efficiencies (>1%)4. Since their initial invention, THz QCLs have been developed for imaging and sensing applications. However, the demanding cooling requirement for THz QCLs has confined this technology in a laboratory environment. Therefore, raising the maximum operating temperature Tmax to above that of a compact cooler (>235 K for a single-stage TEC), or no cooler at all, has been the “holy grail” of the field. The high power portable THz system described here is enabled by a record setting of Tmax = 250 K, far exceeding the previous record of 210 K established in 2019,5 which was an improvement from a 7-year old record of 200 K.6 The new record also far exceeds a decade-old record of 225 K, which was achieved with the aid of a strong magnetic field up to 30 T.7 The significant improvement in Tmax is achieved based on a novel and unique design strategy to achieve a clean 3-level system in the THz QCL structure, and identifying correct parameters for the band structure and a novel optimization scheme for the design. To emphasize the significant advancement and challenge of this work, historical record of Tmax of THz QCLs over time is plotted in Fig. 1. It is clear from Fig. 1 that after the initial fast rise of Tmax in the early years (2002-2005), the progress slowed down and then even stalled after 2012 at 200 K until 2019. This long stretch of lack of progress has prompted many observers to speculate that there is a fundamental physical limit that THz QCLs cannot be operated significantly above 200 K. Our work showed that there is no such a limit, and the result should jolt the field and give the THz community the optimism that high-temperature (even at and above room-temperature) operation of THz QCLs can be achieved. Figure 1:  Experimentally achieved Tmax over time. The temperature of a single-stage thermoelectric cooler (~235 K) and room temperature of 300 K are also marked as reference. The dashed line is for visual guidance. In summary, the significance of our work is two-fold. First, it enables portable THz imaging and spectral systems that will have an immediate impact on wide-range applications in medical, biochemistry, security, etc. Second and perhaps more importantly in the long term, the new design strategy will lead to a further increase of Tmax up to room temperature and beyond.