IT alignment in public organisations is considered to be one of the missing pieces in the otherwise rich research area in the IS domain. Researchers also criticise the lack of studies addressing practical issues related to IT alignment, particularly in public organisations. One such area of concern is the contradictory findings of prior studies on the role of formal and informal organisations on IT alignment. A case study was conducted in one public organisation to establish the influence of different forms of organisational structure on IT alignment. A theoretical model was proposed based on a previous study and literature, which is then tested with data collected from a survey among 163 IT and administrative personnel in a Swedish public organisation. The results suggest that centralised organisational structure as well as the two forms of informal organisational structures—interpersonal relationships and cross-departmental relationships influence IT alignment. However, the influence of professional networks on IT alignment is not significant. The findings are important for researchers and practitioners in public organisations.