Background Sexually transmitted infections (STIs) such as chlamydia are common among young people and can lead to serious health issues if untreated. Although condoms are recommended for prevention, many young people report inconsistent use during penetrative sex. Web-based STI testing is becoming increasingly popular, but these services typically offer minimal support or guidance on preventing future infections. The “Wrapped” intervention aims to help young users of web-based STI testing use condoms consistently and correctly during penetrative sex, thus reducing future STI incidence. Objective This study aims to assess whether and how it is possible to conduct a future randomized controlled trial (RCT) of the Wrapped intervention. Methods Users of web-based STI testing aged 16 years to 24 years were randomized to an online, double-blind, 2-arm, parallel-group feasibility RCT in which Wrapped plus usual care (basic information on STIs and condom use) was tested against usual care alone. Main outcome measures were the proportion of the sampling pool recruited and return of valid chlamydia self-samples at month (M)12. Other outcome measures included return of valid chlamydia self-samples at M3; online survey completion at baseline, M3, M6, and M12; follow-up by demographic characteristics; and acceptability of intervention and measures. Results Over 31weeks, 173 participants were recruited and provided a baseline chlamydia test result, representing 1.5% of the sampling pool (173/11,413; intervention: n=84; control: n=89). A valid chlamydia self-sample was returned by 75.7% (131/173; 95% CI 68.6-81.9) at M12. Therefore, 3574 participants, derived from a sampling pool of 238,266 service users, were estimated to be necessary to power a future full trial. Return of other follow-up measures included 75.1% (130/173) valid M3 chlamydia self-samples, 91.3% (158/173) M3 survey, 90.8% (157/173) M6 survey, and 90.8% (159/173) M12 survey. Participants at M12 appeared to broadly represent individuals in the sampling pool with some exceptions: a tendency for over-representation of participants who were older (20-24 years), of Black ethnicity, and in the least deprived quintile and under-representation of participants who were younger (16-19 years), male, and in deprivation quintile three. There was some evidence that attrition was patterned by ethnicity and age in ways that compounded initial recruitment patterns. Drop-out attrition was evident, with retention higher at M12 for the intervention group (72/84, 86%) than the control group (59/89, 66%). Eleven adverse events relating to participation were reported. A priori criteria for success were met. Conclusions A full trial is feasible. Although the recruitment rate was low, the high volume of young people using web-based STI testing services (approximately 585,000 annually based on the latest data) provides a sufficient pool to meet the required sample size. To ensure balanced representation, strategies to address potential under- and over-representation of certain demographic subgroups by M12 should be implemented. Trial Registration ISRCTN Registry ISRCTN17478654; http://www.isrctn.com/ISRCTN17478654 International Registered Report Identifier (IRRID) RR2-10.2196/43645