Background/Objectives: Patients with severe asthma (SA) commonly present with coexisting nasal polyposis (NP), often requiring treatment with intranasal corticosteroids (INC). However, adherence to INC in this population remains inadequately characterized despite its clinical significance. This study aimed to evaluate adherence to INC in patients with SA and NP and to identify clinical and pharmacological factors associated with adherence levels. Methods: We conducted a retrospective observational study including adult patients with SA and NP treated with INC and followed at a tertiary asthma unit in Madrid during 2024. Adherence was assessed via medication possession ratio (MPR) over six months, with poor adherence defined as MPR < 50%. Pharmacological, clinical and demographic variables were analyzed for associations with adherence. Results: Of the 188 patients evaluated, 86 (45.7%) were prescribed INC. Poor adherence was observed in 53.5% of these patients. Women exhibited significantly lower adherence compared to men (p < 0.05). Fluticasone was the most commonly prescribed INC (54.6%), with no significant adherence differences across corticosteroid types. Patients on maintenance systemic corticosteroids had higher adherence (85.7%, p < 0.05), whereas those receiving biologic therapies tended toward lower adherence (51% poor adherence), though this was not statistically significant. Higher adherence was associated with increased disease severity, as indicated by multiple endoscopic sinus surgeries (p < 0.05). No significant differences were observed in spirometry or Asthma Control Test scores. Conclusions: Adherence to INC in patients with SA and NP is suboptimal, particularly among women and patients on biologics. Greater disease severity correlates with improved adherence. Targeted interventions are necessary to enhance adherence and optimize disease management in this population.