Purpose <p>The standard of care for glioblastoma (GBM) involves maximal-safe resection. We evaluated the relationship between postoperative diffusion-weighted imaging (DWI) changes, extent of resection (EOR), and clinical outcomes in patients with GBM.</p> Methods <p>We retrospectively analyzed 323 patients with newly diagnosed IDH-wildtype GBM who underwent surgical resection between 2005 and 2023. EOR was categorized according to RANO–<i>resect</i> categories into supramaximal (class 1), complete (class 2&#xa0;A), near-total (class 2B), and submaximal (class 3) groups. We assessed postoperative DWI and ADC maps 24–72&#xa0;h following surgery.</p> Results <p>Postoperative DWI restriction was detected in 161 patients and was positively correlated with preoperative tumor volume (<i>r</i> = 0.196,<i>p</i> = 0.026). The most common DWI restriction patterns included sector-shaped (59%), rim-shaped (34%), mixed (6%), and remote (1%). DWI restriction incidence was comparable across resection classes: 37.5% in supramaximal resection, 51% in complete resection, 50.9% in near-total resection, and 50.7% in submaximal resection (<i>p</i> = 0.663). Patients who underwent maximal/supramaximal (class 1 and 2) resection had a significantly longer median OS (20 versus 14 months, <i>p</i> = 0.013;multivariate HR = 0.74[95%CI = 0.58–0.96], <i>p</i> = 0.023) and achieved a favorable Karnofsky Performance Score (KPS) ≥ 70 (87.5% vs. 73.0%) over submaximal (class 3) resection (<i>p</i> = 0.001). New neurological deficits were rare in both the maximal/supramaximal (class 1 and 2) (1.7%) and submaximal (class 3) (4.9%) groups (<i>p</i> = 0.278). DWI restriction did not impact survival (<i>p</i> = 0.499).</p> Conclusion <p>Maximal and supramaximal resections in IDH-wildtype GBM improve survival and function, with low morbidity despite DWI changes. Further research should assess the long-term impact of DWI changes on cognition and quality of life.</p>