Background New methods of dietary assessment are increasingly making use of online technologies. The development of a new online food frequency questionnaire warranted investigation of its feasibility and the reproducibility of its results. Objective To investigate the feasibility and reproducibility of a newly developed online FFQ (WebFFQ). Design The semiquantitative WebFFQ was designed to assess the habitual diet the previous year, with questions about frequency of intake and portion sizes. Estimations of portion sizes include both pictures and household measures, depending on the type of food in question. In two independent cross-sectional studies conducted in 2015 and 2016, adults were recruited by post following random selection from the general population. In the first study, participants ( n = 229) filled in the WebFFQ and answered questions about its feasibility, and in two subsequent focus group meetings, participants ( n = 9) discussed and gave feedback about the feasibility of the WebFFQ. In the second study, the WebFFQ’s reproducibility was assessed by asking participants ( n = 164) to fill it in on two separate occasions, 12 weeks apart. Moreover, in the second study, participants were offered personal dietary feedback, a monetary gift certificate, or both, as incentives to complete the study. Results In the feasibility study, evaluation form results showed that participants raised issues regarding the estimation of portion size and the intake of seasonal foods as being particularly challenging; furthermore, in the focus group discussions, personal feedback on diet was perceived to be a more motivating factor than monetary reward. In the reproducibility study, total food intake was lower in the second WebFFQ; however, 63% of the food groups were not significantly different from those in the first WebFFQ. Correlations of food intake ranged from 0.62 to 0.90, >86% of the participants were classified into the same or adjacent quartiles, and misclassification ranged from 0 to 3%. Average energy intake was 3.5% lower ( p = 0.001), fiber showed the least difference at 1.6% ( p = 0.007), and sugar intake differed the most at −6.8% (borderline significant, p = 0.08). Percentage energy obtained from macronutrients did not differ significantly between the first and second WebFFQs. Conclusion Our results suggest that at group level, the WebFFQ showed good reproducibility for the estimations of intake of food groups, energy, and nutrients. The feasibility of the WebFFQ is good; however, revisions to further improve portion size estimations should be included in future versions. The WebFFQ is considered suitable for dietary assessments for healthy adults in the Norwegian population.