Objective: The aim of this study was to assess a risk-adapted strategy for stage I seminoma guided by the presence of rete testis invasion. Methods: Between January 2013 and December 2015, a total of 135 consecutive patients with stage I seminoma from 18 Spanish tertiary hospitals were included in a prospective multicenter study. Median patient age was 38 years (range 22–60). Preoperative beta-human chorionic gonadotropin was elevated in 9.6% of patients. Rete testis invasion was present in 47.4% of patients. After orchiectomy, subjects with rete testis invasion were treated with 2 courses of adjuvant carboplatin (area under the curve of 7, with 21-day interval). Those without this risk factor were managed by surveillance. Disease-free survival (DFS) and overall survival (OS) were estimated with the Kaplan-Meier method. Results: After a median follow-up time of 33 months, only 6 relapses were recorded (5 on surveillance, 1 after carboplatin). These cases were rescued with BEP or EP chemotherapy, and all 135 patients are currently disease free without sequelae. Three-year DFS was 92.0 and 98.2% for patients on surveillance and after carboplatin, respectively. Three-year OS was 100%. Conclusion: A risk-adapted approach based on rete testis invasion as a single risk factor is feasible and yielded an excellent outcome with a 3-year DFS of 94.9%.