Insomnia is prevalent, emphasizing the need for effective and sustainable treatments. While short-term use of sleep medication is recommended, long-term use remains common, underscoring the necessity for psychological treatments like Cognitive Behavioral Therapy for Insomnia (CBT-I) in clinical practice. This study aimed to evaluate the effectiveness of guided Internet-Based Cognitive Behavioral Therapy for insomnia (ICBT-I) when integrated into general practice. Participants (n = 177) were recruited from 33 primary health care centers (PCCs) and enrolled in an eight-week guided ICBT-I program. Eligible participants were at least 18 years old and reported sleep problems significantly affecting their daily lives. Significant reductions in insomnia were observed, with large improvements in sleep disturbances. ISI scores decreased significantly from pre- to post-treatment (β = 9.368, p < .001, Hedges’ g = 1.40). Depression (β = 5.496, g = 0.68) and anxiety (β = 3.982, g = 0.56) also showed moderate improvements (p < .001). All sleep diary measures improved significantly (p < .001), and sleep medication use dropped from 48.6% at pretreatment to 17.5% at posttreatment (p < .001). These findings suggest that guided ICBT-I in primary care effectively reduces insomnia and improves mental health, with outcomes comparable to specialized care.