Relationships between allergies and cancer are complex and depend on various factors. However, little is known regarding the effects of weight change after smoking cessation on the risk of cancer in patients with allergic diseases. This study investigated 26,614 patients with asthma, allergic rhinitis, and atopic dermatitis from the NHIS-HEALS cohort, who had biennial health screenings between 2005 and 2008. All patients were followed up from 2009 until the date of cancer diagnosis, death, or 2013, whichever earliest. Allergic diseases patients who quit smoking and gained weight have a significantly higher cancer risk than non-smokers with stable weight (adjusted hazard ratio [aHR], 1.59; 95% confidence interval [CI], 1.11–2.27). This risk is particularly elevated in older adults (≥ 65 years, aHR, 2.06; 95% CI, 1.29–3.29), men (aHR, 1.53; 95% CI, 1.05–2.22), and those with multiple comorbidities (aHR, 1.77; 95% CI, 1.24–2.52). Moreover, varying effects of weight gain on lung cancer (aHR, 2.28; 95% CI, 1.21–4.28). These findings highlight the importance of personalized weight management strategies to maximize the benefits of smoking cessation in patients with allergic diseases. Comprehensive public health strategies that address both smoking cessation and weight control may be essential to reduce cancer risk among patients with allergic diseases.