Objective
To examine decade-long temporal trends, seasonal patterns, and sex- and age-specific differences in self-harm notifications and suicide mortality among children and adolescents in Santa Catarina, southern Brazil.

Methods
We conducted a time-series study using data from the Brazilian Notifiable Diseases Information System and the Mortality Information System. Self-harm notifications and suicide deaths among individuals aged 5–19 years between July 2014 and June 2024 were included. Sociodemographic and event-related characteristics were described. Incidence rates and incidence rate ratios (IRR) were calculated by sex and age group. Temporal trends and seasonality were assessed using seasonal and trend decomposition using loess.

Results
A total of 15,550 self-harm notifications were recorded, predominantly among adolescents aged 15–19 years (67.5%) and females (74.8%). The overall incidence of self-harm notifications was 87.8 per 1,000,000 inhabitants, with higher rates among females (IRR 3.12; 95% CI: 3.01–3.24). Notifications increased steadily over the decade and showed moderate seasonality, with peaks between September and November. During the same period, 434 suicide deaths were recorded, mainly among males (68.4%) and adolescents aged 15–19 years (83.2%). Suicide mortality among individuals aged 10–19 years was 3.63 per 1,000,000, higher in males (IRR 2.07; 95% CI: 1.70–2.54). Suicide mortality remained relatively stable and showed weak seasonal variation.

Conclusions
Self-harm notifications increased substantially over the past decade and displayed clear seasonal patterns, whereas suicide mortality remained comparatively stable. Marked differences by sex and age underscore the need for developmentally informed, sex-sensitive prevention strategies and strengthened youth mental health surveillance in Brazil.