Purpose <p>This study aimed to evaluate the impact of nonunion on patient-reported outcomes (PROs) following transforaminal lumbar interbody fusion (TLIF). As the influence of fusion status on clinical outcomes remains controversial, we conducted a retrospective analysis using propensity score matching to adjust for potential confounders affecting postoperative outcomes in nonunion cases.</p> Methods <p>We retrospectively reviewed 415 patients who underwent single-level TLIF for lumbar degenerative disease (LDD) with a minimum 2-year follow-up. Patients were categorized into nonunion (N, <i>n</i> = 119) and union (U, <i>n</i> = 296) groups based on 1-year postoperative CT findings. PROs were assessed using the visual analog scale (VAS) and Japanese Orthopedic Association Back Pain Evaluation Questionnaire (JOABPEQ) scores. Propensity score matching was performed to control baseline differences, yielding 97 matched pairs.</p> Results <p>Before matching, the N group was older, predominantly male, and exhibited worse preoperative sagittal alignment compared to the U group. In the matched cohort, the N group had less improvement in JOABPEQ scores, especially in the domains of low back pain (27.5 vs. 40.4; <i>p</i> = 0.018), lumbar function (17.4 vs. 27.9; <i>p</i> = 0.033), walking ability (34.6 vs. 46.7; <i>p</i> = 0.011) and social life function (25.5 vs. 34.3; <i>p</i> = 0.016), with significantly lower rates of achieving the MCID in lumbar function (<i>p</i> = 0.022) and social life function (<i>p</i> = 0.039).</p> Conclusion <p>Nonunion after single-level TLIF was associated with inferior improvement in lumbar function and social life function. These findings highlight the clinical importance of achieving solid fusion to optimize postoperative recovery.</p>