diff --git a/.changelog/10559.txt b/.changelog/10559.txt
new file mode 100644
index 000000000..9e755940f
--- /dev/null
+++ b/.changelog/10559.txt
@@ -0,0 +1,3 @@
+```release-note:bug
+api: Fix default values used for optional fields in autopilot configuration update (POST to `/v1/operator/autopilot/configuration`) [[GH-10558](https://github.com/hashicorp/consul/issues/10558)]
+```
diff --git a/agent/operator_endpoint.go b/agent/operator_endpoint.go
index f8f2f4fcc..644fe7fba 100644
--- a/agent/operator_endpoint.go
+++ b/agent/operator_endpoint.go
@@ -217,7 +217,7 @@ func (s *HTTPHandlers) OperatorAutopilotConfiguration(resp http.ResponseWriter,
 		s.parseDC(req, &args.Datacenter)
 		s.parseToken(req, &args.Token)
 
-		var conf api.AutopilotConfiguration
+		conf := api.NewAutopilotConfiguration()
 		if err := decodeBody(req.Body, &conf); err != nil {
 			return nil, BadRequestError{Reason: fmt.Sprintf("Error parsing autopilot config: %v", err)}
 		}
diff --git a/agent/operator_endpoint_test.go b/agent/operator_endpoint_test.go
index ae22e9705..37f893b44 100644
--- a/agent/operator_endpoint_test.go
+++ b/agent/operator_endpoint_test.go
@@ -435,7 +435,21 @@ func TestOperator_AutopilotSetConfiguration(t *testing.T) {
 	a := NewTestAgent(t, "")
 	defer a.Shutdown()
 
+	// Provide a non-default value only for CleanupDeadServers.
+	// Expect all other fields to be updated with default values
+	// (except CreateIndex and ModifyIndex).
 	body := bytes.NewBuffer([]byte(`{"CleanupDeadServers": false}`))
+	expected := structs.AutopilotConfig{
+		CleanupDeadServers:      false, // only non-default value
+		LastContactThreshold:    200 * time.Millisecond,
+		MaxTrailingLogs:         250,
+		MinQuorum:               0,
+		ServerStabilizationTime: 10 * time.Second,
+		RedundancyZoneTag:       "",
+		DisableUpgradeMigration: false,
+		UpgradeVersionTag:       "",
+	}
+
 	req, _ := http.NewRequest("PUT", "/v1/operator/autopilot/configuration", body)
 	resp := httptest.NewRecorder()
 	if _, err := a.srv.OperatorAutopilotConfiguration(resp, req); err != nil {
@@ -453,9 +467,11 @@ func TestOperator_AutopilotSetConfiguration(t *testing.T) {
 	if err := a.RPC("Operator.AutopilotGetConfiguration", &args, &reply); err != nil {
 		t.Fatalf("err: %v", err)
 	}
-	if reply.CleanupDeadServers {
-		t.Fatalf("bad: %#v", reply)
-	}
+
+	// For equality comparison check, ignore CreateIndex and ModifyIndex
+	expected.CreateIndex = reply.CreateIndex
+	expected.ModifyIndex = reply.ModifyIndex
+	require.Equal(t, expected, reply)
 }
 
 func TestOperator_AutopilotCASConfiguration(t *testing.T) {
diff --git a/api/operator_autopilot.go b/api/operator_autopilot.go
index 8175f5133..d713ee1cc 100644
--- a/api/operator_autopilot.go
+++ b/api/operator_autopilot.go
@@ -58,6 +58,23 @@ type AutopilotConfiguration struct {
 	ModifyIndex uint64
 }
 
+// Defines default values for the AutopilotConfiguration type, consistent with
+// https://www.consul.io/api-docs/operator/autopilot#parameters-1
+func NewAutopilotConfiguration() AutopilotConfiguration {
+	cfg := AutopilotConfiguration{
+		CleanupDeadServers:      true,
+		LastContactThreshold:    NewReadableDuration(200 * time.Millisecond),
+		MaxTrailingLogs:         250,
+		MinQuorum:               0,
+		ServerStabilizationTime: NewReadableDuration(10 * time.Second),
+		RedundancyZoneTag:       "",
+		DisableUpgradeMigration: false,
+		UpgradeVersionTag:       "",
+	}
+
+	return cfg
+}
+
 // ServerHealth is the health (from the leader's point of view) of a server.
 type ServerHealth struct {
 	// ID is the raft ID of the server.