In the context of coping with chronic illness, shared appraisal refers to one person’s perception that the illness is a shared issue. Despite increasing research linking shared appraisal to positive relationship and health outcomes, whether it is static or fluctuates across time has not yet been investigated. Fluctuations in shared appraisal may affect patient and spouse well-being and influence disease management. We investigated the association of daily shared appraisal fluctuations (operationalized as intraindividual standard deviations) to relationship satisfaction, psychological well-being, and diabetes-specific well-being over 2 weeks among 199 couples in which one person had type 1 diabetes. In addition, we examined attachment insecurity as a correlate of shared appraisal fluctuations. Individuals higher in anxiety or avoidance may more frequently reevaluate their relationship. Thus, their shared appraisal may be more reactive to daily events. Actor–partner interdependence models showed greater fluctuations in spouse appraisal were associated with lower patient relationship satisfaction (partner effect) but not with spouse relationship satisfaction and with greater spouse psychological distress (actor effect) but not patient psychological distress. There were no actor or partner effects of patient appraisal fluctuations on these outcomes. In terms of diabetes outcomes, regression analyses showed that greater fluctuations in patient and spouse appraisal were each associated with higher patient diabetes distress, and greater fluctuations in spouse appraisal were associated with lower patient self-care. Higher attachment anxiety was related to greater appraisal fluctuations in the spouse but not the patient. These findings highlight the importance of spouse appraisal stability for spouse relationships and well-being and for patient diabetes outcomes and suggest that attachment insecurity may be implicated in the instability of shared appraisal.