diff --git a/internal/cmd/params_validation.go b/internal/cmd/params_validation.go
index 2274b3f80..a55316b18 100644
--- a/internal/cmd/params_validation.go
+++ b/internal/cmd/params_validation.go
@@ -26,7 +26,7 @@ func buildStartValidationInput(ctx *Context, args []string) (core.StartParamInpu
 		if argsLenAtDash >= len(args) {
 			return core.StartParamInput{}, nil
 		}
-		return core.StartParamInput{DashArgs: args[argsLenAtDash:]}, nil
+		return core.StartParamInput{DashArgs: quoteStartDashArgs(args[argsLenAtDash:])}, nil
 	}
 
 	raw, err := ctx.Command.Flags().GetString("params")
diff --git a/internal/cmd/start.go b/internal/cmd/start.go
index 1a630e2f2..9f0772b08 100644
--- a/internal/cmd/start.go
+++ b/internal/cmd/start.go
@@ -340,7 +340,9 @@ func loadDAGWithParams(ctx *Context, args []string, isSubDAGRun bool) (*core.DAG
 	var params string
 
 	if ctx.Command.ArgsLenAtDash() != -1 && len(args) > 0 {
-		loadOpts = append(loadOpts, spec.WithParams(args[ctx.Command.ArgsLenAtDash():]))
+		dashArgs := args[ctx.Command.ArgsLenAtDash():]
+		loadOpts = append(loadOpts, spec.WithParams(quoteStartDashArgs(dashArgs)))
+		params = strings.Join(dashArgs, " ")
 	} else {
 		params, err = ctx.Command.Flags().GetString("params")
 		if err != nil {
diff --git a/internal/cmd/start_params.go b/internal/cmd/start_params.go
new file mode 100644
index 000000000..56e61dbcd
--- /dev/null
+++ b/internal/cmd/start_params.go
@@ -0,0 +1,37 @@
+// Copyright (C) 2026 Yota Hamada
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package cmd
+
+import (
+	"encoding/json"
+	"strings"
+
+	"github.com/dagucloud/dagu/internal/cmn/stringutil"
+	"github.com/dagucloud/dagu/internal/core/spec"
+)
+
+func quoteStartDashArgs(args []string) []string {
+	if isSingleJSONDashArg(args) {
+		return args
+	}
+	return spec.QuoteRuntimeParams(args, nil)
+}
+
+func isSingleJSONDashArg(args []string) bool {
+	if len(args) != 1 {
+		return false
+	}
+
+	input := strings.TrimSpace(stringutil.RemoveQuotes(args[0]))
+	if input == "" {
+		return false
+	}
+
+	isObject := strings.HasPrefix(input, "{") && strings.HasSuffix(input, "}")
+	isArray := strings.HasPrefix(input, "[") && strings.HasSuffix(input, "]")
+	if !isObject && !isArray {
+		return false
+	}
+	return json.Valid([]byte(input))
+}
diff --git a/internal/cmd/start_test.go b/internal/cmd/start_test.go
index c9bf35455..a671e33c2 100644
--- a/internal/cmd/start_test.go
+++ b/internal/cmd/start_test.go
@@ -36,6 +36,11 @@ steps:
   - name: "1"
     run: "echo \"params is $1 and $2\""
 `)
+	dagStartWithSingleParam := th.DAG(t, `params: "p1"
+steps:
+  - name: "1"
+    run: "echo \"params is $1\""
+`)
 
 	dagStartWithDAGRunID := th.DAG(t, `steps:
   - name: "1"
@@ -63,6 +68,11 @@ steps:
 			Args:        []string{"start", dagStartWithParams.Location, "--", "p5", "p6"},
 			ExpectedOut: []string{`params="[1=p5 2=p6`},
 		},
+		{
+			Name:        "StartDAGWithSpacedParamAfterDash",
+			Args:        []string{"start", dagStartWithSingleParam.Location, "--", "Something here"},
+			ExpectedOut: []string{`params="[1=Something here]"`},
+		},
 		{
 			Name:        "StartDAGWithRequestID",
 			Args:        []string{"start", dagStartWithDAGRunID.Location, "--run-id", "CfmC9GPywTC24bXbY1yEU7eQANNvpdxAPJXdSKTSaCVC"},
@@ -204,6 +214,14 @@ steps:
 			Args: []string{"start", dagFile, "--", `{"KEY":"value"}`},
 		})
 		require.NoError(t, err)
+
+		dag, err := spec.Load(th.Context, dagFile)
+		require.NoError(t, err)
+
+		status, err := th.DAGRunMgr.GetLatestStatus(th.Context, dag)
+		require.NoError(t, err)
+		require.Equal(t, core.Succeeded, status.Status)
+		require.Contains(t, status.Params, "KEY=value")
 	})
 
 	t.Run("AllowsNamedPairsWhenNoParamsDeclared", func(t *testing.T) {