Based on an expressway project in Shanghai, the structural behavior, construction procedure, and engineering applicability of a novel longitudinally split precast segmental cap beam were investigated. The proposed cap beam is divided longitudinally into two precast segments for lifting and erection. After erection, the segments are connected by a cast-in-place wet joint placed on a specially designed bottom formwork, while the cap beam is connected to the columns through grouted sleeve connections. For the two-column portal-frame cap beam commonly used in urban viaducts, two tendon-layout schemes were developed. In Scheme 1, all the prestressing tendons are arranged within the precast segments; the wet joint is 500 mm wide, and the single-lift weight is 143 t. In Scheme 2, some of the prestressing tendons are arranged within the wet joint; the joint width is 700 mm, and the single-lift weight is 126 t, approximately 12% lower than that of Scheme 1. The results show that the wet-joint width is strongly coupled with the single-lift weight, available construction space, and tendon arrangement and is therefore a key parameter governing the applicability of the system. Compared with the conventional transversely segmented precast cap beam, the proposed longitudinally split scheme increases tendon consumption by approximately 20% and requires more cast-in-place work but reduces the single-lift weight by approximately 30% and eliminates temporary support and traffic diversion, minimizing disruption to under-bridge traffic. The proposed system therefore demonstrates good engineering applicability and provides technical support for the prefabricated and rapid construction of urban viaducts.