The research was conducted as a retrospective cohort study. For a study, researchers sought to determine how obesity affected postoperative outcomes and the accomplishment of the minimum clinically relevant difference (MCID) after minimally invasive transforaminal lumbar interbody fusion (MIS TLIF). For primary, elective, single-level MIS TLIF patients, a prospective surgical database was analyzed retrospectively. Patients were divided into 4 BMI groups: nonobese (<30 kg/m2), obesity I (≥30 and <35 kg/m2), severe (≥35 and <40 kg/m2), and morbid (≥40 kg/m2). The groups were compared in demographics, perioperative information, and complication rates. Preoperatively and up to 2-year postoperatively, VAS back and leg, Oswestry Disability Index (ODI), 12-Item Short Form-12 Physical Composite Score (SF-12 PCS), and PROMIS PF were obtained. At all time points, the impact of BMI on outcome indicators and MCID attainment was assessed. A total of 162 patients were enrolled in the study, with 88 being of normal weight, 37 being obese I, 25 being severe, and 12 being morbid. BMI significantly influenced SF-12 PCS and PROMIS PF at all timepoints, but only at 6- and 12-weeks for VAS back, preoperatively and 12 weeks for VAS leg, and preoperatively to 1-year for ODI. At 2 years, MCID achievement differed only for PROMIS PF and VAS but not for overall MCID achievement. BMI was a significant predictor of ODI, SF-12 PCS, and PROMIS PF scores from preoperative to 1-year postoperatively, but only for VAS scores through 12-week. Although BMI may affect postoperative outcome values, obesity may affect a patient’s sense of significant pain and physical function improvements, according to MCID accomplishment for PROMIS PF and VAS back.