Background: Psychological pain (PP) is a potentially important risk factor for suicide. However, its temporal stability and association with suicidal ideation (SI) remain obscure. Furthermore, the relation between PP and depression and hopelessness is understudied. As SI may intensively oscillate and affective instability and PP are common experiences in borderline personality disorder (BPD), we also examined factors predicting short-term changes of SI in depressive inpatients with or without BPD. Methods: Psychiatric inpatients with depression without (N=37) and with BPD (N=30) were recruited to an ecological momentary assessment (EMA) study, wherein their PP, severity of depression, SI, and hopelessness were assessed three times daily using visual analogue scales during the hospitalization period. Multilevel regression models were estimated. Results: Altogether 4320 EMA observations were collected. PP correlated with hopelessness (r = 0.417), depression (r = 0.339), and anxiety (r = 0.496), but between-patient variance of PP remained at 1.26 (95% CI = 1.025–1.533) after controlling for these variables. Patients with depression and BPD reported higher daily PP and SI (p<0.001) and more prominent within-patient variation of PP. Depression (? = 0.19; [95% CI = 0.06, 0.32]) and previous SI (? = 0.32; [95% CI = 0.23, 0.41]) predicted near-future SI within several hours. PP predicted near-future SI in inpatients with depression and BPD (? = 0.28; [95% CI = 0.11, 0.46]), but not in patients without BPD. Conclusions: Besides depression and hopelessness, PP represents an independent risk factor for SI. Depressive patients with BPD may experience more prominent and temporally unstable PP. Short-terms changes in SI are predicted by changes in severity of depression and previous SI in depressed inpatients without BPD, and also by changes in PP in inpatients with BPD. As SI and its risk factors may oscillate within a time scale of hours, frequent monitoring of momentary severity of depression, PP, and SI may be warranted in inpatient settings.