Objective To study the clinical characteristics of rashes induced by trimethoprim-sulfamethoxazole (TMP-SMZ) in children treated for pertussis and to inform safe medication practices. Methods A retrospective analysis was conducted on 238 children diagnosed with pertussis and treated with TMP-SMZ at Wuhu First People's Hospital from January to August 2024. The incidence and clinical features of rashes were summarized. Results Of 238 children, 34 (14.3%) developed rashes; 19 (55.9%) were boys, and the 5 to <10-year age group accounted for the highest proportion (70.6%, 24/34). A history of allergic disease was present in 50.0% (17/34). Rashes typically appeared on or after day 7 of therapy (82%, 28/34) and were predominantly erythematous or maculopapular eruptions (97%, 33/34); 71% (24/34) were pruritic. Fever occurred in 56% (19/34); among those who were tested for respiratory viruses, 77% (10/13) were positive for viruses such as rhinovirus and adenovirus. After discontinuation of TMP-SMZ, rashes resolved within 3 days in 97% (33/34) of patients (41% within 1 day; 56% within more than 1 but within 3 days). There was no significant difference in rash incidence between photoprotection and non-photoprotection groups (P>0.05). Conclusions TMP-SMZ for pertussis can induce rashes, particularly in children aged 5 to <10 years. The eruption is usually a pruritic erythematous or maculopapular rash, with over half of cases accompanied by fever and frequent concomitant viral infections. Most rashes resolve within 3 days after drug withdrawal. The potential association between the rash and sun exposure warrants further investigation.