The loss of a significant other may shatter positive or neutral cognitions about the self, life, and the future, confirm preexisting negative cognitions in these areas, or lead to threatening misinterpretations about one's own grieving. Negative cognitions are a risk factor for the development of prolonged grief disorder (PGD). Using latent trait-state occasion (TSO) modeling, the aim of this study was to examine the longitudinal structure of negative loss-related cognitions over time, and to examine the associations between time-invariant (TI; trait) and time-variant (TV; occasional) components of four types of negative cognitions with symptoms of prolonged grief (PG). Data from the Utrecht Longitudinal Study on Adjustment to Loss were used, in which bereaved people's grief responses were measured every 6 weeks over the course of 1 year (11 waves). We included 223 participants who experienced their loss in the year before. Using TSO modeling, we could distinguish between the degree of variance in negative cognitions explained by the stable trait factor (TI component) and by temporary, situation-dependent circumstances (TV component). Descriptive results indicated a decrease in both negative cognitions and PG scores over time. While the TSO model for negative cognitions about the self showed inadequate fit to the data, the other models showed that negative cognitions about life, the future, and threatening misinterpretations largely consisted of a TI component, which was more strongly associated with PG severity compared to the TV components. Thus, negative views on life, pessimism over the future, and negative interpretations of one's grief reactions seem to be relatively stable over time and less sensitive to changing circumstances. This supports the potential usefulness of cognitive restructuring, behavioral experiments, and exposure in the treatment of PG. Overall, this study confirms the role of negative cognitions in the development and maintenance of symptoms of PG.