To address the issues of water–air cross-contamination, liquid droplet carryover, and high maintenance costs of traditional water-mediated evaporative cooling systems, the hollow fiber membrane integrated water cooler (MWC) is proposed as a novel liquid cooling solution. The MWC incorporates hydrophobic porous membranes that enable water vapor migration while avoiding direct contact between liquid water and air. In addition, the MWC offers advantages over conventional evaporative cooling paddings in terms of high specific surface area, corrosion resistance, and anti-scaling. To evaluate the cooling performance of a countercurrent MWC, an experimental bench was built to test its outlet water temperature under various operating conditions. The developed numerical model was verified with the measured data. Then, the influence of nine critical parameters, including operating conditions and membrane module specifications, on the outlet water temperature and cooling efficiency of the countercurrent MWC was numerically investigated. The results show that evaporative heat dissipation contributes to more than 80% of the total heat exchange in MWC. Cold and dry ambient air, high inlet water temperature, low water velocity, and high air/water ratio contribute to superior cooling capability. Moreover, the cooling performance of MWC improves with increasing fiber length, inner diameter, packing fraction, and decreasing membrane thickness. In addition, the MWC offers a cooling capacity per unit volume of up to 1094.72 kW/m3, an order of magnitude greater than conventional wet cooling towers. Overall, this work presents theoretical guidelines for optimizing the operating conditions and configuration of countercurrent MWCs.