Approximately 9% of people think about suicide during their lifetime. Suicidal thoughts are consistently associated with perceived failures in emotion regulation. However, factors contributing to these perceptions remain insufficiently clear. New evidence suggests that when people know little about the cause of their emotions (i.e., low source attribution of emotion), they perceive themselves as less successful in regulating them. Therefore, emotion regulation deficits in people with suicidal thoughts might be related to lower knowledge about sources of emotions. We examined this question in two ecological momentary assessment studies (N₁ = 396, N₂ = 195). We found that participants with current suicidal thoughts knew less about the sources of their emotions compared to participants with no suicidal thoughts history (Studies 1 and 2), and even when compared to controls with similar levels of psychiatric symptoms but no history of suicidal thoughts (Study 2). Using language processing, we found that written descriptions of the source of participants' emotions were less concrete among those with suicidal thoughts compared to participants with no suicidal thoughts history. Among suicidal participants, suicidal thoughts were more likely to be present in moments when participants knew less than usual about the source of their negative emotions (Study 2), and low knowledge of the source was associated with more frequent and prolonged suicidal thoughts (Studies 1 and 2). Finally, lower perceived success in emotion regulation mediated the association between source attribution of emotion and the occurrence of suicidal thoughts. Findings suggest that reduced knowledge about the source of negative emotions might increase the risk for suicidal thinking. (PsycInfo Database Record (c) 2024 APA, all rights reserved).