Efforts to reduce sugar intake levels have been primarily limited to increasing knowledge and changing attitudes. We conducted a systematic review to (1) identify factors influencing adults' knowledge and attitudes about sugar, and (2) determine if there is an association between knowledge and attitudes about sugar and sugar intake. We searched 15 electronic databases from inception to December 2016 for English language publications including adults with relevant exposure and outcome measures. Findings were summarised meta-narratively. Of 3287 studies, 22 studies (14 for objective one and 8 for objective two) were included. Individual (liking of sugary food), interpersonal (attitudes of peers) and environmental factors (media, health professionals and food labelling) influenced adults’ knowledge and attitudes about sugar, at least to some extent. Overall, quality of the studies included in our review was weak, and evidence for the application of the Knowledge–Attitude–Behavior model for understanding sugar intake is limited. Protocol registered in the PROSPERO International prospective register of systematic reviews (registration number CRD42015027540).