This article reports the development and preliminary testing of a new scale named "Partnership Scale for Primary Family Caregivers Caring for Patients With Dementia" that measures the ability of primary family caregivers to establish partnerships while providing care for patients with dementia in Japan. The first draft of the scale was developed using qualitative data from interviews with five primary family caregivers; a pool of 39 items was created through a review process with dementia care experts and researchers. An exploratory factor analysis and confirmatory factor analysis were conducted with data from 261 primary family caregivers who completed the instrument. This resulted in a multidimensional scale that consists of three factors with 13 items. The suitability of the model and intraclass correlation coefficient (ICC) values (1, 1) obtained by the test–retest method satisfied statistical standards. The criterion-related validity of the scale was significantly correlated to an external reference, which was the desired outcome. However, some subscales exhibited low internal consistency, demonstrating the need for further research.