Undermining is one of the most challenging complications of deep pressure ulcers. Recommendations in most guidelines are based only on expert opinions. Here, we examined the relationship between surgical incision of the undermined space and pressure ulcer healing through a Japanese multicenter prospective cohort study. A total of 162 patients with undermining in 40 national hospitals in Japan were enrolled from July 2007 to June 2009. The incision group included 39 patients (24.1%) whose undermining was surgically incised during the observational period. Their 4-week follow-up data on pressure ulcer severity and areas of healthy granulation tissue were recorded as outcome variables using the DESIGN-R pressure ulcer assessment tool. The 4-week follow-up was restarted after the incision in the incision group. The outcome variables over time were compared between the two groups using a linear mixed model with or without adjustment for demographic and other variables. The incision group showed more rapid improvement in the total and granulation DESIGN-R scores compared with the nonincision group (p < 0.001 and p = 0.007, respectively, in the crude models). This study may provide the first considerable evidence to support that surgical incision of undermining may promote healing of deep pressure ulcers.