Background: Recreational sun exposure has been associated with melanoma prevalence, and tourism settings are of particular interest for skin cancer prevention. Effective, affordable, and geographically flexible interventions to promote sun protection are needed. Objective: The aim of this study was to describe the protocol for a definitive randomized controlled trial (RCT) evaluating a smartphone mobile intervention (mISkin app) promoting sun protection in holidaymakers and to assess the acceptability and feasibility of the mISkin app and associated trial procedures in an internal pilot study. Methods: Participants were recruited from the general community. Holidaymakers traveling abroad and owning a smartphone were enrolled in the internal pilot of a 2 (mISkin vs control) x 2 (sun protection factor [SPF] 15 vs SPF 30) RCT with a postholiday follow-up. The smartphone app is fully automated and entails a behavioral intervention to promote sun protection. It consisted of five components: skin assessment, educational videos, ultraviolet (UV) photos, gamification, and prompts for sun protection. Participants were also randomly allocated to receive sunscreen SPF 15 or SPF 30. Primary outcomes for the internal pilot study were acceptability and feasibility of trial procedures and intervention features. Secondary outcomes were collected at baseline and after holidays through face-to-face-assessments and included skin sun damage, sunscreen use (residual weight and application events), and sun protection practices (Web-based questionnaire). Results: From 142 registers of interest, 42 participants were randomized (76% [32/42] female; mean age 35.5 years). Outcome assessments were completed by all participants. Random allocation to SPF 15 versus SPF 30 was found not to be feasible in a definitive trial protocol. Of the 21 people allocated to the mISkin intervention, 19 (91%) installed the mISkin on their phones, and 18 (86%) used it at least once. Participants were satisfied with the mISkin app and made suggestions for further improvements. Due to difficulties with the random allocation to SPF and slow uptake, the trial was discontinued. Conclusions: The internal pilot study concluded that randomization to SPF was not feasible and that recruitment rate was slower than expected because of difficulties with gatekeeper engagement. Possible solutions to the problems identified are discussed. Further refinements to the mISkin app are needed before a definitive trial. Trial Registration: International Standard Randomized Controlled Trial Number ISRCTN63943558; http://www.isrctn.com/ISRCTN63943558 (Archived by WebCite at http://www.webcitation.org/6xOLvbab8) [JMIR Dermatol 2018;1(1):e1]