diff --git a/server/events/yaml/parser_validator_test.go b/server/events/yaml/parser_validator_test.go
index bc3f2e42a1..11df9f9b1b 100644
--- a/server/events/yaml/parser_validator_test.go
+++ b/server/events/yaml/parser_validator_test.go
@@ -206,7 +206,7 @@ projects:
 						WorkflowName:     nil,
 						TerraformVersion: nil,
 						Autoplan: valid.Autoplan{
-							WhenModified: []string{"**/*.tf*"},
+							WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 							Enabled:      true,
 						},
 						ApplyRequirements: nil,
@@ -220,6 +220,28 @@ projects:
 			input: `
 version: 3
 projects:
+- dir: "."
+`,
+			exp: valid.RepoCfg{
+				Version: 3,
+				Projects: []valid.Project{
+					{
+						Dir:       ".",
+						Workspace: "default",
+						Autoplan: valid.Autoplan{
+							WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
+							Enabled:      true,
+						},
+					},
+				},
+				Workflows: make(map[string]valid.Workflow),
+			},
+		},
+		{
+			description: "autoplan should be enabled by default if only when_modified set",
+			input: `
+version: 3
+projects:
 - dir: "."
   autoplan:
     when_modified: ["**/*.tf*"]
@@ -253,7 +275,7 @@ projects:
 						Dir:       ".",
 						Workspace: "default",
 						Autoplan: valid.Autoplan{
-							WhenModified: []string{"**/*.tf*"},
+							WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 							Enabled:      true,
 						},
 					},
@@ -276,7 +298,7 @@ workflows: ~
 						Dir:       ".",
 						Workspace: "default",
 						Autoplan: valid.Autoplan{
-							WhenModified: []string{"**/*.tf*"},
+							WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 							Enabled:      true,
 						},
 					},
@@ -304,7 +326,7 @@ workflows:
 						Dir:       ".",
 						Workspace: "default",
 						Autoplan: valid.Autoplan{
-							WhenModified: []string{"**/*.tf*"},
+							WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 							Enabled:      true,
 						},
 					},
@@ -339,7 +361,7 @@ workflows:
 						WorkflowName:     String("myworkflow"),
 						TerraformVersion: tfVersion,
 						Autoplan: valid.Autoplan{
-							WhenModified: []string{"**/*.tf*"},
+							WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 							Enabled:      true,
 						},
 						ApplyRequirements: []string{"approved"},
@@ -377,7 +399,7 @@ workflows:
 						WorkflowName:     String("myworkflow"),
 						TerraformVersion: tfVersion,
 						Autoplan: valid.Autoplan{
-							WhenModified: []string{"**/*.tf*"},
+							WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 							Enabled:      false,
 						},
 						ApplyRequirements: []string{"approved"},
@@ -415,7 +437,7 @@ workflows:
 						WorkflowName:     String("myworkflow"),
 						TerraformVersion: tfVersion,
 						Autoplan: valid.Autoplan{
-							WhenModified: []string{"**/*.tf*"},
+							WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 							Enabled:      false,
 						},
 						ApplyRequirements: []string{"mergeable"},
@@ -453,7 +475,7 @@ workflows:
 						WorkflowName:     String("myworkflow"),
 						TerraformVersion: tfVersion,
 						Autoplan: valid.Autoplan{
-							WhenModified: []string{"**/*.tf*"},
+							WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 							Enabled:      false,
 						},
 						ApplyRequirements: []string{"mergeable", "approved"},
@@ -567,7 +589,7 @@ projects:
 						Dir:       ".",
 						Workspace: "workspace",
 						Autoplan: valid.Autoplan{
-							WhenModified: []string{"**/*.tf*"},
+							WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 							Enabled:      true,
 						},
 					},
@@ -576,7 +598,7 @@ projects:
 						Dir:       ".",
 						Workspace: "workspace",
 						Autoplan: valid.Autoplan{
-							WhenModified: []string{"**/*.tf*"},
+							WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 							Enabled:      true,
 						},
 					},
@@ -608,7 +630,7 @@ workflows:
 						Dir:       ".",
 						Workspace: "default",
 						Autoplan: valid.Autoplan{
-							WhenModified: []string{"**/*.tf*"},
+							WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 							Enabled:      true,
 						},
 					},
@@ -670,7 +692,7 @@ workflows:
 						Dir:       ".",
 						Workspace: "default",
 						Autoplan: valid.Autoplan{
-							WhenModified: []string{"**/*.tf*"},
+							WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 							Enabled:      true,
 						},
 					},
@@ -728,7 +750,7 @@ workflows:
 						Dir:       ".",
 						Workspace: "default",
 						Autoplan: valid.Autoplan{
-							WhenModified: []string{"**/*.tf*"},
+							WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 							Enabled:      true,
 						},
 					},
@@ -782,7 +804,7 @@ workflows:
 						Dir:       ".",
 						Workspace: "default",
 						Autoplan: valid.Autoplan{
-							WhenModified: []string{"**/*.tf*"},
+							WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 							Enabled:      true,
 						},
 					},
diff --git a/server/events/yaml/raw/autoplan.go b/server/events/yaml/raw/autoplan.go
index 49b0a95eab..33f0b1fa1a 100644
--- a/server/events/yaml/raw/autoplan.go
+++ b/server/events/yaml/raw/autoplan.go
@@ -1,10 +1,12 @@
 package raw
 
-import "github.com/runatlantis/atlantis/server/events/yaml/valid"
+import (
+	"github.com/runatlantis/atlantis/server/events/yaml/valid"
+)
 
 // DefaultAutoPlanWhenModified is the default element in the when_modified
 // list if none is defined.
-const DefaultAutoPlanWhenModified = "**/*.tf*"
+var DefaultAutoPlanWhenModified = []string{"**/*.tf*", "**/terragrunt.hcl"}
 
 type Autoplan struct {
 	WhenModified []string `yaml:"when_modified,omitempty"`
@@ -14,7 +16,7 @@ type Autoplan struct {
 func (a Autoplan) ToValid() valid.Autoplan {
 	var v valid.Autoplan
 	if a.WhenModified == nil {
-		v.WhenModified = []string{DefaultAutoPlanWhenModified}
+		v.WhenModified = DefaultAutoPlanWhenModified
 	} else {
 		v.WhenModified = a.WhenModified
 	}
@@ -35,7 +37,7 @@ func (a Autoplan) Validate() error {
 // DefaultAutoPlan returns the default autoplan config.
 func DefaultAutoPlan() valid.Autoplan {
 	return valid.Autoplan{
-		WhenModified: []string{DefaultAutoPlanWhenModified},
+		WhenModified: DefaultAutoPlanWhenModified,
 		Enabled:      valid.DefaultAutoPlanEnabled,
 	}
 }
diff --git a/server/events/yaml/raw/autoplan_test.go b/server/events/yaml/raw/autoplan_test.go
index 4e29007c91..66744340df 100644
--- a/server/events/yaml/raw/autoplan_test.go
+++ b/server/events/yaml/raw/autoplan_test.go
@@ -109,7 +109,7 @@ func TestAutoplan_ToValid(t *testing.T) {
 			input:       raw.Autoplan{},
 			exp: valid.Autoplan{
 				Enabled:      true,
-				WhenModified: []string{"**/*.tf*"},
+				WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 			},
 		},
 		{
@@ -129,7 +129,7 @@ func TestAutoplan_ToValid(t *testing.T) {
 			},
 			exp: valid.Autoplan{
 				Enabled:      false,
-				WhenModified: []string{"**/*.tf*"},
+				WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 			},
 		},
 		{
@@ -139,7 +139,7 @@ func TestAutoplan_ToValid(t *testing.T) {
 			},
 			exp: valid.Autoplan{
 				Enabled:      true,
-				WhenModified: []string{"**/*.tf*"},
+				WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 			},
 		},
 	}
diff --git a/server/events/yaml/raw/project_test.go b/server/events/yaml/raw/project_test.go
index 626f09b54f..c11580bbcc 100644
--- a/server/events/yaml/raw/project_test.go
+++ b/server/events/yaml/raw/project_test.go
@@ -231,7 +231,7 @@ func TestProject_ToValid(t *testing.T) {
 				WorkflowName:     nil,
 				TerraformVersion: nil,
 				Autoplan: valid.Autoplan{
-					WhenModified: []string{"**/*.tf*"},
+					WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 					Enabled:      true,
 				},
 				ApplyRequirements: nil,
@@ -276,7 +276,7 @@ func TestProject_ToValid(t *testing.T) {
 				Workspace:        "default",
 				TerraformVersion: tfVersionPointEleven,
 				Autoplan: valid.Autoplan{
-					WhenModified: []string{"**/*.tf*"},
+					WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 					Enabled:      true,
 				},
 			},
@@ -291,7 +291,7 @@ func TestProject_ToValid(t *testing.T) {
 				Dir:       ".",
 				Workspace: "default",
 				Autoplan: valid.Autoplan{
-					WhenModified: []string{"**/*.tf*"},
+					WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 					Enabled:      true,
 				},
 			},
@@ -319,7 +319,7 @@ func TestProject_ToValid(t *testing.T) {
 				Dir:       "mydir",
 				Workspace: "default",
 				Autoplan: valid.Autoplan{
-					WhenModified: []string{"**/*.tf*"},
+					WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 					Enabled:      true,
 				},
 			},
@@ -334,7 +334,7 @@ func TestProject_ToValid(t *testing.T) {
 				Dir:       "mydir",
 				Workspace: "default",
 				Autoplan: valid.Autoplan{
-					WhenModified: []string{"**/*.tf*"},
+					WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 					Enabled:      true,
 				},
 			},
@@ -392,7 +392,7 @@ func TestProject_ToValid(t *testing.T) {
 				Dir:       ".",
 				Workspace: "default",
 				Autoplan: valid.Autoplan{
-					WhenModified: []string{"**/*.tf*"},
+					WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 					Enabled:      true,
 				},
 			},
diff --git a/server/events/yaml/raw/repo_cfg_test.go b/server/events/yaml/raw/repo_cfg_test.go
index 5d3da46d86..c199ba980b 100644
--- a/server/events/yaml/raw/repo_cfg_test.go
+++ b/server/events/yaml/raw/repo_cfg_test.go
@@ -355,7 +355,7 @@ func TestConfig_ToValid(t *testing.T) {
 						Dir:       "mydir",
 						Workspace: "default",
 						Autoplan: valid.Autoplan{
-							WhenModified: []string{"**/*.tf*"},
+							WhenModified: []string{"**/*.tf*", "**/terragrunt.hcl"},
 							Enabled:      true,
 						},
 					},