<b><i>Objectives:</i></b> Bariatric surgery is the most effective weight loss therapy, and recently laparoscopic sleeve gastrectomy (LSG) is gaining popularity worldwide. On the other hand, patients undergoing bariatric surgery have a high prevalence of mental disorders. A Japanese nationwide survey reported high prevalence of mental disorders in patients with low percent total weight loss (%TWL) and also in those with high %TWL. The aim of this study was to investigate the relationship of 1-year %TWL with background mental health status, 3-year outcomes, and nutrition intake in Japanese patients after LSG. <b><i>Methods:</i></b> This study was a single-center retrospective database analysis. A total of 89 Japanese patients who underwent LSG and were followed for 3 years were enrolled (mean age 41.9 years, baseline body mass index 44.9, baseline glycosylated hemoglobin, HbA<sub>1c</sub>, 7.0%). The patients were divided into 3 groups according to 1-year %TWL as follows: ≤19.9% (insufficient group), 20.0–34.9% (average group) and ≥35.0% (excessive group). Psychosocial and nutritional status as well as physical data were collected from all patients. <b><i>Results:</i></b> The prevalence of mental disorders was 51.7%, and 1-year %TWL was 28.1% in all patients. No significant differences were observed in the changes in body weight and HbA<sub>1c</sub> between patients with and those without mental disorders. The prevalence of mental disorders was particularly high in the insufficient and excessive groups. In the insufficient group, mood disorders and mental retardation/developmental disorders were frequent, and snacking and eating out habits were often observed. In the excessive group, the frequencies of mood disorders and binge eating were high, and a decrease in skeletal muscle mass due to low protein intake was observed. Furthermore, weight regain was shown 12 months after LSG in both groups. In the average group, there were fewer problems in weight loss outcomes, mental health, nutrition intake and body composition. <b><i>Conclusions:</i></b> Psychosocial and nutritional problems were often found not only in patients with insufficient weight loss, but also in those with seemingly “excellent” weight reduction. To improve long-term weight loss outcome and future health, a multidisciplinary approach focusing on mental health and nutrition is essential for patients undergoing bariatric surgery.