Regulatory noncompliance is a major challenge globally for fisheries sustainability. Estimating noncompliance and understanding its motivations is important for developing strategies to reduce illegality. In Chile, the socio-ecologically important kelp fishery (Lessonia spicata; L. berteorana) is suspected of having high levels of noncompliance. We estimated the levels and types of noncompliance within the fishery directly from small-scale fishers, along with exploring motivations for noncompliance. We did so across four different harvesting regulations, two management regimes, and two geographies. Levels of noncompliance differed across all three factors. Fishers have higher compliance in areas managed with territorial user rights management regimes compared to those managed by regional management plans that establish control rules. However, the differences were more pronounced in one geography compared to another. Fishers show high levels of normative and legitimacy-based motivations with respect to complying with kelp harvesting regulations. In contrast, instrumental motivations were weaker and more heterogenous, which may be tied to noncompliance. Results support the premise that a lack of enforcement may be the main driver of noncompliance in the Chilean kelp fishery. Thus, deterrent mechanisms such as enhancing the perceived likelihood of detection, improving surveillance methods, increasing penalties, and strengthening co-management governance through combining informal and formal mechanisms may help increase compliance.