Introduction Flexible flatfeet are commonly treated with functional foot orthoses (FFOs) and University of California Berkeley Laboratory (UCBL) designs. This study sought to evaluate the immediate effects of low-profile FFOs and high-profile UCBL orthoses on Foot Posture Index (FPI), spatiotemportal measures of gait, and perceived comfort and compliance in adults with flexible flatfeet. Materials and Methods Fourteen individuals with self-reported asymptomatic flexible flatfeet participated in this randomized cross-sectional study. Custom FFOs and UCBL orthoses were fabricated for each participant, who then walked at a self-selected speed in each condition on a Zeno Mat. Foot Posture Index, perceived comfort and compliance, and spatiotemporal parameters were collected across conditions. Results There were no significant differences in velocity, cadence, step length, stride width, toe-in/toe-out, or stance center of pressure distance percentage. Stance phase duration significantly increased from baseline to UCBL condition. There was a statistically significant decrease bilaterally in FPI from baseline to FFO and from baseline to UCBL orthosis, but not between FFO and UCBL orthosis. There were no significant differences in comfort and compliance ratings between FFOs and UCBL orthoses. Conclusions With regard to immediate effects, both the UCBL and FFO designs improve foot posture without significant impact on spatiotemporal gait patterns or perceived comfort and compliance. Additional research is needed to determine the longevity of these effects. Clinical Relevance The results of this study support the use of both FFOs and UCBL orthoses when trying to provide foot posture correction for adults with asymptomatic flexible flatfeet.