Developing and sustaining partnerships for promoting health has been identified as an important strategy for addressing the health challenges that face society. Trust is one of the most important factors that help partnerships function effectively. In health promotion partnerships, trust is an under-researched and poorly understood phenomenon. This study was designed to identify how trust is conceptualized in health promotion partnerships and to develop a trust measurement tool. Five focus groups were organized with 36 health promotion partners in order to explore how trust is conceptualized in their partnerships. Participants represented health, community, education, arts, sports and youth sectors. A content analysis was carried out on the transcripts and a 14-item, five-point scale, was developed from the findings. This scale was incorporated into an overall questionnaire on partnership functioning which was posted to 469 partners in 40 health promotion partnerships. A response rate of 72% was achieved (n = 337) for the postal survey. The trust scale was subjected to reliability and validity tests. Principal Component Analysis yielded two components, named positive trust and mistrust, explaining 59% of the variance. Coefficients ranged from 0.845 to 0.511 with eigen-values before rotation of 6.58 and 1.66. Cronbach's alpha was 0.91. Further research is required to establish whether the scale can be used with other types of partnerships.