Although nonsuicidal self-injury (NSSI), by definition, excludes suicidal intent, numerous studies show associations between NSSI and suicidal phenomena in clinical and outpatient adolescent samples. Given the growing interest in the relationship between NSSI and suicidal phenomena, the present study aimed to investigate the relationship between NSSI and suicidal beliefs in adolescent psychiatric inpatients. The study sample included 50 adolescent inpatients at a specialized facility, with a mean age of 15.44 ± 1.39, who fulfilled DSM-5 criteria for NSSI. For study purposes, we use the Ottawa Self-Injury Inventory (OSI) and Brief Suicide Cognitions Scale (B-SCS). Statistical data processing was performed in the R software 4.3.0 (R Core Team, Vienna, Austria). : Of all NSSI functions, the Internal ER function score was the highest (18.72 ± 7.08), followed by External ER (8.10 ± 3.11), Social Influence (5.88 ± 5.37), and Sensation Seeking (3.44 ± 2.98). The mean Craving (C) score was 14.06 ± 7.51. The mean value of the B-SCS score was 19.54 ± 5.24. It was found that the B-SCS score is significantly related to Internal ER (r = 0.441, < 0.001) and Craving (r = 0.297, = 0.036). The multivariable model shows that internal ER function and participants' age are significantly related to the B-SCS score. : Despite the limitations of the study, it is emphasized that cognitions occurring across the fluid suicidal belief system alone do not fully capture the complexity of suicide, but assessing the suicidal belief system in NSSI inpatient adolescents could nevertheless provide helpful information for identifying individuals who may have an elevated vulnerability to experiencing suicidal ideas and behaviors over time.