diff --git a/hugolib/hugo_sites.go b/hugolib/hugo_sites.go
index 6f95dbb12..5be4ec02b 100644
--- a/hugolib/hugo_sites.go
+++ b/hugolib/hugo_sites.go
@@ -62,6 +62,9 @@ type HugoSites struct {
 	// If this is running in the dev server.
 	running bool
 
+	// Serializes rebuilds when server is running.
+	runningMu sync.Mutex
+
 	// Render output formats for all sites.
 	renderFormats output.Formats
 
diff --git a/hugolib/hugo_sites_build.go b/hugolib/hugo_sites_build.go
index d748a0169..7f725def2 100644
--- a/hugolib/hugo_sites_build.go
+++ b/hugolib/hugo_sites_build.go
@@ -31,6 +31,12 @@ import (
 // Build builds all sites. If filesystem events are provided,
 // this is considered to be a potential partial rebuild.
 func (h *HugoSites) Build(config BuildCfg, events ...fsnotify.Event) error {
+	if h.running {
+		// Make sure we don't trigger rebuilds in parallel.
+		h.runningMu.Lock()
+		defer h.runningMu.Unlock()
+	}
+
 	ctx, task := trace.NewTask(context.Background(), "Build")
 	defer task.End()