diff --git a/helmfiles/values/prometheus.yaml.gotmpl b/helmfiles/values/prometheus.yaml.gotmpl
index 27645ba0f60d79cc53176ebc74ee4518531b7031..4f05cd179e3fe0e6f1aa1c0456686109f7fa854b 100644
--- a/helmfiles/values/prometheus.yaml.gotmpl
+++ b/helmfiles/values/prometheus.yaml.gotmpl
@@ -7,6 +7,7 @@ kubeDns:
 
 alertmanager:
   alertmanagerSpec:
+    logFormat: logfmt
     storage:
       volumeClaimTemplate:
         metadata: