This study explored how the use of smartphones can influence sports betting by young adults, compared to using computers and land-based betting facilities. Interviews with 33 Australians aged 18–29 years, who bet regularly on sports, esports, and/or fantasy sports, were analysed using adaptive grounded theory. Seven major themes related to platform functionality, sourcing betting information, physical accessibility, financial accessibility, social influences, privacy, and marketing. The grounded theory model depicts how features of smartphones, online gambling, and betting apps combine in smartphone betting to provide instantaneous access to betting, anywhere and at any time, to facilitate harmful betting behaviours. These behaviours included increased betting participation, frequency and expenditure, placing a wider variety of bets, impulsive and spontaneous betting, placing riskier bets with longer odds, chasing losses, and acting on social encouragement to bet. These findings can inform harm minimisation measures, regulation, and policy.