diff --git a/src/builder/action.rs b/src/builder/action.rs
index ff5890a51ea..fa1ccd57eef 100644
--- a/src/builder/action.rs
+++ b/src/builder/action.rs
@@ -119,6 +119,8 @@ pub enum ArgAction {
     IncOccurrence,
     /// When encountered, act as if `"true"` was encountered on the command-line
     ///
+    /// If no [`default_value`][super::Arg::default_value] is set, it will be `false`.
+    ///
     /// No value is allowed
     ///
     /// # Examples
@@ -133,17 +135,27 @@ pub enum ArgAction {
     ///             .action(clap::builder::ArgAction::SetTrue)
     ///     );
     ///
-    /// let matches = cmd.try_get_matches_from(["mycmd", "--flag", "--flag"]).unwrap();
+    /// let matches = cmd.clone().try_get_matches_from(["mycmd", "--flag", "--flag"]).unwrap();
     /// assert!(matches.is_present("flag"));
     /// assert_eq!(matches.occurrences_of("flag"), 0);
     /// assert_eq!(
     ///     matches.get_one::<bool>("flag").copied(),
     ///     Some(true)
     /// );
+    ///
+    /// let matches = cmd.try_get_matches_from(["mycmd"]).unwrap();
+    /// assert!(matches.is_present("flag"));
+    /// assert_eq!(matches.occurrences_of("flag"), 0);
+    /// assert_eq!(
+    ///     matches.get_one::<bool>("flag").copied(),
+    ///     Some(false)
+    /// );
     /// ```
     SetTrue,
     /// When encountered, act as if `"false"` was encountered on the command-line
     ///
+    /// If no [`default_value`][super::Arg::default_value] is set, it will be `true`.
+    ///
     /// No value is allowed
     ///
     /// # Examples
@@ -158,17 +170,27 @@ pub enum ArgAction {
     ///             .action(clap::builder::ArgAction::SetFalse)
     ///     );
     ///
-    /// let matches = cmd.try_get_matches_from(["mycmd", "--flag", "--flag"]).unwrap();
+    /// let matches = cmd.clone().try_get_matches_from(["mycmd", "--flag", "--flag"]).unwrap();
     /// assert!(matches.is_present("flag"));
     /// assert_eq!(matches.occurrences_of("flag"), 0);
     /// assert_eq!(
     ///     matches.get_one::<bool>("flag").copied(),
     ///     Some(false)
     /// );
+    ///
+    /// let matches = cmd.try_get_matches_from(["mycmd"]).unwrap();
+    /// assert!(matches.is_present("flag"));
+    /// assert_eq!(matches.occurrences_of("flag"), 0);
+    /// assert_eq!(
+    ///     matches.get_one::<bool>("flag").copied(),
+    ///     Some(true)
+    /// );
     /// ```
     SetFalse,
     /// When encountered, increment a `u64` counter
     ///
+    /// If no [`default_value`][super::Arg::default_value] is set, it will be `0`.
+    ///
     /// No value is allowed
     ///
     /// # Examples
@@ -183,13 +205,21 @@ pub enum ArgAction {
     ///             .action(clap::builder::ArgAction::Count)
     ///     );
     ///
-    /// let matches = cmd.try_get_matches_from(["mycmd", "--flag", "--flag"]).unwrap();
+    /// let matches = cmd.clone().try_get_matches_from(["mycmd", "--flag", "--flag"]).unwrap();
     /// assert!(matches.is_present("flag"));
     /// assert_eq!(matches.occurrences_of("flag"), 0);
     /// assert_eq!(
     ///     matches.get_one::<u64>("flag").copied(),
     ///     Some(2)
     /// );
+    ///
+    /// let matches = cmd.try_get_matches_from(["mycmd"]).unwrap();
+    /// assert!(matches.is_present("flag"));
+    /// assert_eq!(matches.occurrences_of("flag"), 0);
+    /// assert_eq!(
+    ///     matches.get_one::<u64>("flag").copied(),
+    ///     Some(0)
+    /// );
     /// ```
     Count,
     /// When encountered, display [`Command::print_help`][super::App::print_help]
@@ -246,6 +276,20 @@ pub enum ArgAction {
 }
 
 impl ArgAction {
+    pub(crate) fn default_value(&self) -> Option<&'static std::ffi::OsStr> {
+        match self {
+            Self::Set => None,
+            Self::Append => None,
+            Self::StoreValue => None,
+            Self::IncOccurrence => None,
+            Self::SetTrue => Some(std::ffi::OsStr::new("false")),
+            Self::SetFalse => Some(std::ffi::OsStr::new("true")),
+            Self::Count => Some(std::ffi::OsStr::new("0")),
+            Self::Help => None,
+            Self::Version => None,
+        }
+    }
+
     pub(crate) fn takes_value(&self) -> bool {
         match self {
             Self::Set => true,
diff --git a/src/builder/arg.rs b/src/builder/arg.rs
index 9f62e06241c..6f9fc767887 100644
--- a/src/builder/arg.rs
+++ b/src/builder/arg.rs
@@ -4892,6 +4892,11 @@ impl<'help> Arg<'help> {
             self.settings.set(ArgSettings::TakesValue);
         }
         if let Some(action) = self.action.as_ref() {
+            if let Some(default_value) = action.default_value() {
+                if self.default_vals.is_empty() {
+                    self.default_vals = vec![default_value];
+                }
+            }
             if action.takes_value() {
                 self.settings.set(ArgSettings::TakesValue);
             } else {
diff --git a/tests/builder/action.rs b/tests/builder/action.rs
index 25abb1de4f1..1697914d84a 100644
--- a/tests/builder/action.rs
+++ b/tests/builder/action.rs
@@ -81,10 +81,10 @@ fn set_true() {
         Command::new("test").arg(Arg::new("mammal").long("mammal").action(ArgAction::SetTrue));
 
     let matches = cmd.clone().try_get_matches_from(["test"]).unwrap();
-    assert_eq!(matches.get_one::<bool>("mammal"), None);
-    assert_eq!(matches.is_present("mammal"), false);
+    assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), false);
+    assert_eq!(matches.is_present("mammal"), true);
     assert_eq!(matches.occurrences_of("mammal"), 0);
-    assert_eq!(matches.index_of("mammal"), None);
+    assert_eq!(matches.index_of("mammal"), Some(1));
 
     let matches = cmd
         .clone()
@@ -106,7 +106,7 @@ fn set_true() {
 }
 
 #[test]
-fn set_true_with_default_value() {
+fn set_true_with_explicit_default_value() {
     let cmd = Command::new("test").arg(
         Arg::new("mammal")
             .long("mammal")
@@ -141,6 +141,10 @@ fn set_true_with_default_value_if_present() {
         )
         .arg(Arg::new("dog").long("dog").action(ArgAction::SetTrue));
 
+    let matches = cmd.clone().try_get_matches_from(["test"]).unwrap();
+    assert_eq!(*matches.get_one::<bool>("dog").unwrap(), false);
+    assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), false);
+
     let matches = cmd.clone().try_get_matches_from(["test", "--dog"]).unwrap();
     assert_eq!(*matches.get_one::<bool>("dog").unwrap(), true);
     assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), true);
@@ -149,7 +153,7 @@ fn set_true_with_default_value_if_present() {
         .clone()
         .try_get_matches_from(["test", "--mammal"])
         .unwrap();
-    assert_eq!(matches.get_one::<bool>("dog"), None);
+    assert_eq!(*matches.get_one::<bool>("dog").unwrap(), false);
     assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), true);
 }
 
@@ -164,6 +168,10 @@ fn set_true_with_default_value_if_value() {
         )
         .arg(Arg::new("dog").long("dog").action(ArgAction::SetTrue));
 
+    let matches = cmd.clone().try_get_matches_from(["test"]).unwrap();
+    assert_eq!(*matches.get_one::<bool>("dog").unwrap(), false);
+    assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), false);
+
     let matches = cmd.clone().try_get_matches_from(["test", "--dog"]).unwrap();
     assert_eq!(*matches.get_one::<bool>("dog").unwrap(), true);
     assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), true);
@@ -172,7 +180,7 @@ fn set_true_with_default_value_if_value() {
         .clone()
         .try_get_matches_from(["test", "--mammal"])
         .unwrap();
-    assert_eq!(matches.get_one::<bool>("dog"), None);
+    assert_eq!(*matches.get_one::<bool>("dog").unwrap(), false);
     assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), true);
 }
 
@@ -191,12 +199,12 @@ fn set_true_with_required_if_eq() {
         .clone()
         .try_get_matches_from(["test", "--mammal"])
         .unwrap();
-    assert_eq!(matches.get_one::<u64>("dog"), None);
+    assert_eq!(*matches.get_one::<bool>("dog").unwrap(), false);
     assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), true);
 
-    cmd.clone()
-        .try_get_matches_from(["test", "--dog"])
-        .unwrap_err();
+    let matches = cmd.clone().try_get_matches_from(["test", "--dog"]).unwrap();
+    assert_eq!(*matches.get_one::<bool>("dog").unwrap(), true);
+    assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), false);
 
     let matches = cmd
         .clone()
@@ -215,10 +223,10 @@ fn set_false() {
     );
 
     let matches = cmd.clone().try_get_matches_from(["test"]).unwrap();
-    assert_eq!(matches.get_one::<bool>("mammal"), None);
-    assert_eq!(matches.is_present("mammal"), false);
+    assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), true);
+    assert_eq!(matches.is_present("mammal"), true);
     assert_eq!(matches.occurrences_of("mammal"), 0);
-    assert_eq!(matches.index_of("mammal"), None);
+    assert_eq!(matches.index_of("mammal"), Some(1));
 
     let matches = cmd
         .clone()
@@ -240,7 +248,7 @@ fn set_false() {
 }
 
 #[test]
-fn set_false_with_default_value() {
+fn set_false_with_explicit_default_value() {
     let cmd = Command::new("test").arg(
         Arg::new("mammal")
             .long("mammal")
@@ -275,6 +283,10 @@ fn set_false_with_default_value_if_present() {
         )
         .arg(Arg::new("dog").long("dog").action(ArgAction::SetFalse));
 
+    let matches = cmd.clone().try_get_matches_from(["test"]).unwrap();
+    assert_eq!(*matches.get_one::<bool>("dog").unwrap(), true);
+    assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), true);
+
     let matches = cmd.clone().try_get_matches_from(["test", "--dog"]).unwrap();
     assert_eq!(*matches.get_one::<bool>("dog").unwrap(), false);
     assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), false);
@@ -283,7 +295,7 @@ fn set_false_with_default_value_if_present() {
         .clone()
         .try_get_matches_from(["test", "--mammal"])
         .unwrap();
-    assert_eq!(matches.get_one::<bool>("dog"), None);
+    assert_eq!(*matches.get_one::<bool>("dog").unwrap(), true);
     assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), false);
 }
 
@@ -298,6 +310,10 @@ fn set_false_with_default_value_if_value() {
         )
         .arg(Arg::new("dog").long("dog").action(ArgAction::SetFalse));
 
+    let matches = cmd.clone().try_get_matches_from(["test"]).unwrap();
+    assert_eq!(*matches.get_one::<bool>("dog").unwrap(), true);
+    assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), true);
+
     let matches = cmd.clone().try_get_matches_from(["test", "--dog"]).unwrap();
     assert_eq!(*matches.get_one::<bool>("dog").unwrap(), false);
     assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), false);
@@ -306,7 +322,7 @@ fn set_false_with_default_value_if_value() {
         .clone()
         .try_get_matches_from(["test", "--mammal"])
         .unwrap();
-    assert_eq!(matches.get_one::<bool>("dog"), None);
+    assert_eq!(*matches.get_one::<bool>("dog").unwrap(), true);
     assert_eq!(*matches.get_one::<bool>("mammal").unwrap(), false);
 }
 
@@ -315,10 +331,10 @@ fn count() {
     let cmd = Command::new("test").arg(Arg::new("mammal").long("mammal").action(ArgAction::Count));
 
     let matches = cmd.clone().try_get_matches_from(["test"]).unwrap();
-    assert_eq!(matches.get_one::<u64>("mammal"), None);
-    assert_eq!(matches.is_present("mammal"), false);
+    assert_eq!(*matches.get_one::<u64>("mammal").unwrap(), 0);
+    assert_eq!(matches.is_present("mammal"), true);
     assert_eq!(matches.occurrences_of("mammal"), 0);
-    assert_eq!(matches.index_of("mammal"), None);
+    assert_eq!(matches.index_of("mammal"), Some(1));
 
     let matches = cmd
         .clone()
@@ -340,7 +356,7 @@ fn count() {
 }
 
 #[test]
-fn count_with_default_value() {
+fn count_with_explicit_default_value() {
     let cmd = Command::new("test").arg(
         Arg::new("mammal")
             .long("mammal")
@@ -375,6 +391,10 @@ fn count_with_default_value_if_present() {
         )
         .arg(Arg::new("dog").long("dog").action(ArgAction::Count));
 
+    let matches = cmd.clone().try_get_matches_from(["test"]).unwrap();
+    assert_eq!(*matches.get_one::<u64>("dog").unwrap(), 0);
+    assert_eq!(*matches.get_one::<u64>("mammal").unwrap(), 0);
+
     let matches = cmd.clone().try_get_matches_from(["test", "--dog"]).unwrap();
     assert_eq!(*matches.get_one::<u64>("dog").unwrap(), 1);
     assert_eq!(*matches.get_one::<u64>("mammal").unwrap(), 10);
@@ -383,7 +403,7 @@ fn count_with_default_value_if_present() {
         .clone()
         .try_get_matches_from(["test", "--mammal"])
         .unwrap();
-    assert_eq!(matches.get_one::<u64>("dog"), None);
+    assert_eq!(*matches.get_one::<u64>("dog").unwrap(), 0);
     assert_eq!(*matches.get_one::<u64>("mammal").unwrap(), 1);
 }
 
@@ -398,9 +418,13 @@ fn count_with_default_value_if_value() {
         )
         .arg(Arg::new("dog").long("dog").action(ArgAction::Count));
 
+    let matches = cmd.clone().try_get_matches_from(["test"]).unwrap();
+    assert_eq!(*matches.get_one::<u64>("dog").unwrap(), 0);
+    assert_eq!(*matches.get_one::<u64>("mammal").unwrap(), 0);
+
     let matches = cmd.clone().try_get_matches_from(["test", "--dog"]).unwrap();
     assert_eq!(*matches.get_one::<u64>("dog").unwrap(), 1);
-    assert_eq!(matches.get_one::<u64>("mammal"), None);
+    assert_eq!(*matches.get_one::<u64>("mammal").unwrap(), 0);
 
     let matches = cmd
         .clone()
@@ -413,6 +437,6 @@ fn count_with_default_value_if_value() {
         .clone()
         .try_get_matches_from(["test", "--mammal"])
         .unwrap();
-    assert_eq!(matches.get_one::<u64>("dog"), None);
+    assert_eq!(*matches.get_one::<u64>("dog").unwrap(), 0);
     assert_eq!(*matches.get_one::<u64>("mammal").unwrap(), 1);
 }