Purpose: The purpose of this study was to present a new absorbable spacer for medial compartmental knee osteoarthritis (OA). The functional and radiographic results of patients treated with the novel surgical technique were also evaluated to investigate its clinical feasibility. Methods: Patients with medial compartmental knee OA who underwent open-wedge high tibial osteotomy (HTO) with absorbable β-TCP/PLGA [30% beta-tricalcium phosphate and 70% poly (lactic-co-glycolic acid)] spacer implantation and proximal fibular osteotomy from January 2016 to February 2017 were retrospectively analyzed. The operation time, blood loss, and relevant complications were reviewed. The femorotibial angle (FTA), varus angle (VA), joint line convergence angle (JLCA), American Knee Society Score (KSS), and visual analog scale (VAS) score were recorded preoperatively and at the final follow-up, respectively. Results: At the final follow-up, the mean FTA and JLCA were 179.85° ± 4.34° and 2.44° ± 1.26°, respectively, which were smaller than the preoperative values (182.94° ± 3.86° and 4.12° ± 1.65°, respectively; both p < 0.001). The final VA and VAS score were lower than those measured preoperatively (both p < 0.001). The clinical and functional KSSs at the final follow-up were higher than the preoperative scores (p < 0.001). Conclusions: This novel absorbable spacer could provide adequate stability for the tibial osteotomy gap, at a lower cost than a traditional plate. With the use of this spacer as an osteoinductive and biodegradable device, secondary surgery for fixation removal could be avoided. The novel surgical technique could improve both the radiographic appearance and the function of the knee in patients with knee OA.