Objective: To evaluate the effects of insoles adapted into flip-flop sandals on pain and function in individuals with plantar fasciopathy (PF). Design: Randomized, double-blind controlled study. Setting: Physiotherapy clinic of the Faculty of Health Sciences of Trairi, Federal University of Rio Grande do Norte, Santa Cruz, Brazil. Subjects: Sixty-six patients of both genders with PF were randomized into two groups: sandal insole group (SI; n = 34), which received a pair of custom flip-flop sandals with insoles covered with smooth synthetic leather; and plain sandal group (PS; n = 32), which received an identical pair of flip-flop sandals, but without the insoles. Interventions: Patients were instructed to wear the flip-flops for 12 weeks for at least 4 hours/day. Main measures: Pain (visual analogue scale—VAS) in the morning and at the end of the day were considered primary outcomes. Function (Foot Function Index—FFI and Foot and Ankle Ability Measure—FAAM) and functional capacity (6-minute walk test—6MWT) were considered secondary outcomes. The outcomes were evaluated at baseline and immediately after the intervention by a blind assessor. Results: Between-group differences were observed in terms of morning pain (mean difference (MD) = –1.82 cm; 95% confidence interval (CI) = –3.3 to −0.3; P = 0.016) and function (MD = –0.10; 95% CI = –0.19 to −0.01; P = 0.023) after the interventions with the SI group showing superior improvements in comparison to the PS group. Conclusion: The use of insoles adapted in flip-flop sandals for 12 weeks was effective at improving pain and function in individuals with PF. Level of evidence: 1b.