This study investigates the nature and behaviour of illegal gamblers in Hong Kong. A face-to-face street survey of 512 gamblers was conducted in Hong Kong between September and December 2015 with supplementary convenience sampling allowing for analysis of a total sample of 103 illegal gamblers. 56% of illegal gamblers recorded results consistent with this study’s definition of ‘excessive gambling’ [i.e. moderate risk and problem gamblers under the Problem Gambling Severity Index (PGSI)]. 81% of surveyed illegal gamblers were male, 77% were aged between 30 and 49 and 67% were in blue collar occupations. Illegal gamblers bet more frequently on both legal and illegal games than their legal counterparts and spent more when they did bet. While this research did not indicate the direction of causality between illegal and excessive gambling, international work (de Bruin et al. in verslingerd aan meer dan een spel: Een onderzoek naar de aard en omvang van kansspelproblematiek in Nederland, WODC/CVO, Utrecht, http://www.lexandgaming.eu/nl/wp-content/uploads/2015/01/Verslingerd-aan-meer-dan-een-spel.pdf, 2005; Binde in What are the most harmful forms of gambling? Analysing problem gambling prevalence surveys, http://www.utbildning.gu.se/digitalAssets/1327/1327132_cefos-wp12.pdf, 2011) suggests that excessive gamblers are drawn to illegal gambling. Reform could allow excessive gambling by illegal gamblers to be better addressed and initial work suggests some financial benefits to Hong Kong.Electronic supplementary materialThe online version of this article (10.1186/s40405-017-0030-7) contains supplementary material, which is available to authorized users.