diff --git a/app.go b/app.go
index 1d22bd1..f30595c 100644
--- a/app.go
+++ b/app.go
@@ -78,6 +78,8 @@ type App struct {
 	CommandNotFound CommandNotFoundFunc
 	// Execute this function if a usage error occurs
 	OnUsageError OnUsageErrorFunc
+	// Execute this function when an unknown flag is accessed from the context
+	UnknownFlagHandler UnknownFlagFunc
 	// Compilation date
 	Compiled time.Time
 	// List of all authors who contributed
diff --git a/context.go b/context.go
index e79ec34..a14a341 100644
--- a/context.go
+++ b/context.go
@@ -46,6 +46,9 @@ func (cCtx *Context) NumFlags() int {
 
 // Set sets a context flag to a value.
 func (cCtx *Context) Set(name, value string) error {
+	if cCtx.flagSet.Lookup(name) == nil {
+		cCtx.onUnknownFlag(name)
+	}
 	return cCtx.flagSet.Set(name, value)
 }
 
@@ -158,7 +161,7 @@ func (cCtx *Context) lookupFlagSet(name string) *flag.FlagSet {
 			return c.flagSet
 		}
 	}
-
+	cCtx.onUnknownFlag(name)
 	return nil
 }
 
@@ -190,6 +193,12 @@ func (cCtx *Context) checkRequiredFlags(flags []Flag) requiredFlagsErr {
 	return nil
 }
 
+func (cCtx *Context) onUnknownFlag(name string) {
+	if cCtx.App != nil && cCtx.App.UnknownFlagHandler != nil {
+		cCtx.App.UnknownFlagHandler(cCtx, name)
+	}
+}
+
 func makeFlagNameVisitor(names *[]string) func(*flag.Flag) {
 	return func(f *flag.Flag) {
 		nameParts := strings.Split(f.Name, ",")
diff --git a/context_test.go b/context_test.go
index 55a9ead..1ffe040 100644
--- a/context_test.go
+++ b/context_test.go
@@ -150,6 +150,19 @@ func TestContext_Value(t *testing.T) {
 	expect(t, c.Value("unknown-flag"), nil)
 }
 
+func TestContext_Value_UnknownFlagHandler(t *testing.T) {
+	set := flag.NewFlagSet("test", 0)
+	var flagName string
+	app := &App{
+		UnknownFlagHandler: func(_ *Context, name string) {
+			flagName = name
+		},
+	}
+	c := NewContext(app, set, nil)
+	c.Value("missing")
+	expect(t, flagName, "missing")
+}
+
 func TestContext_Args(t *testing.T) {
 	set := flag.NewFlagSet("test", 0)
 	set.Bool("myflag", false, "doc")
@@ -258,6 +271,19 @@ func TestContext_Set(t *testing.T) {
 	expect(t, c.IsSet("int"), true)
 }
 
+func TestContext_Set_StrictLookup(t *testing.T) {
+	set := flag.NewFlagSet("test", 0)
+	var flagName string
+	app := &App{
+		UnknownFlagHandler: func(_ *Context, name string) {
+			flagName = name
+		},
+	}
+	c := NewContext(app, set, nil)
+	c.Set("missing", "")
+	expect(t, flagName, "missing")
+}
+
 func TestContext_LocalFlagNames(t *testing.T) {
 	set := flag.NewFlagSet("test", 0)
 	set.Bool("one-flag", false, "doc")
diff --git a/funcs.go b/funcs.go
index 0a9b22c..1342bd2 100644
--- a/funcs.go
+++ b/funcs.go
@@ -23,6 +23,9 @@ type CommandNotFoundFunc func(*Context, string)
 // is displayed and the execution is interrupted.
 type OnUsageErrorFunc func(cCtx *Context, err error, isSubcommand bool) error
 
+// UnknownFlagFunc is executed when an unknown flag is accessed from the context.
+type UnknownFlagFunc func(*Context, string)
+
 // ExitErrHandlerFunc is executed if provided in order to handle exitError values
 // returned by Actions and Before/After functions.
 type ExitErrHandlerFunc func(cCtx *Context, err error)