diff --git a/Cargo.lock b/Cargo.lock
index fe49d19..3c4d2fc 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -51,6 +51,7 @@ name = "fortifier-example"
 version = "0.0.1"
 dependencies = [
  "fortifier",
+ "tokio",
 ]
 
 [[package]]
@@ -213,6 +214,12 @@ version = "2.3.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9b4f627cb1b25917193a259e49bdad08f671f8d9708acfd5fe0a8c1455d87220"
 
+[[package]]
+name = "pin-project-lite"
+version = "0.2.16"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3b3cff922bd51709b605d9ead9aa71031d81447142d828eb4a6eba76fe619f9b"
+
 [[package]]
 name = "potential_utf"
 version = "0.1.4"
@@ -357,6 +364,27 @@ dependencies = [
  "zerovec",
 ]
 
+[[package]]
+name = "tokio"
+version = "1.48.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ff360e02eab121e0bc37a2d3b4d4dc622e6eda3a8e5253d5435ecf5bd4c68408"
+dependencies = [
+ "pin-project-lite",
+ "tokio-macros",
+]
+
+[[package]]
+name = "tokio-macros"
+version = "2.6.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "af407857209536a95c8e56f8231ef2c2e2aff839b22e07a1ffcbc617e9db9fa5"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "toml"
 version = "0.9.8"
diff --git a/Cargo.toml b/Cargo.toml
index 7ca6768..3ebb78f 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -12,6 +12,7 @@ version = "0.0.1"
 [workspace.dependencies]
 fortifier = { path = "./packages/fortifier", version = "0.0.1" }
 fortifier-macros = { path = "./packages/fortifier-macros", version = "0.0.1" }
+tokio = "1.48.0"
 
 [workspace.lints.rust]
 unsafe_code = "deny"
diff --git a/example/Cargo.toml b/example/Cargo.toml
index a33ae5a..9c66441 100644
--- a/example/Cargo.toml
+++ b/example/Cargo.toml
@@ -10,6 +10,7 @@ version.workspace = true
 
 [dependencies]
 fortifier.workspace = true
+tokio = { workspace = true, features = ["macros", "rt-multi-thread"] }
 
 [lints]
 workspace = true
diff --git a/example/src/main.rs b/example/src/main.rs
index c61972c..312c56f 100644
--- a/example/src/main.rs
+++ b/example/src/main.rs
@@ -12,16 +12,32 @@ struct CreateUser {
 
     #[validate(url)]
     url: String,
+
+    #[validate(custom(function = validate_one_locale_required, error = OneLocaleRequiredError))]
+    locales: Vec<String>,
+}
+
+#[derive(Debug)]
+struct OneLocaleRequiredError;
+
+fn validate_one_locale_required(locales: &[String]) -> Result<(), OneLocaleRequiredError> {
+    if locales.is_empty() {
+        Err(OneLocaleRequiredError)
+    } else {
+        Ok(())
+    }
 }
 
-fn main() -> Result<(), Box<dyn Error>> {
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn Error>> {
     let data = CreateUser {
         email: "john@doe.com".to_owned(),
         name: "John Doe".to_owned(),
         url: "https://john.doe.com".to_owned(),
+        locales: vec!["en_GB".to_owned()],
     };
 
-    data.validate_sync()?;
+    data.validate().await?;
 
     Ok(())
 }
diff --git a/packages/fortifier-macros/src/validate/field.rs b/packages/fortifier-macros/src/validate/field.rs
index e722a69..513f337 100644
--- a/packages/fortifier-macros/src/validate/field.rs
+++ b/packages/fortifier-macros/src/validate/field.rs
@@ -4,7 +4,7 @@ use syn::{Field, Result};
 
 use crate::{
     validation::Validation,
-    validations::{Email, Length, Url},
+    validations::{Custom, Email, Length, Url},
 };
 
 pub struct ValidateField {
@@ -22,7 +22,11 @@ impl ValidateField {
         for attr in &field.attrs {
             if attr.path().is_ident("validate") {
                 attr.parse_nested_meta(|meta| {
-                    if meta.path.is_ident("email") {
+                    if meta.path.is_ident("custom") {
+                        result.validations.push(Box::new(Custom::parse(&meta)?));
+
+                        Ok(())
+                    } else if meta.path.is_ident("email") {
                         result.validations.push(Box::new(Email::parse(&meta)?));
 
                         Ok(())
diff --git a/packages/fortifier-macros/src/validations.rs b/packages/fortifier-macros/src/validations.rs
index 2909a91..dbaa948 100644
--- a/packages/fortifier-macros/src/validations.rs
+++ b/packages/fortifier-macros/src/validations.rs
@@ -1,7 +1,9 @@
+mod custom;
 mod email;
 mod length;
 mod url;
 
+pub use custom::*;
 pub use email::*;
 pub use length::*;
 pub use url::*;
diff --git a/packages/fortifier-macros/src/validations/custom.rs b/packages/fortifier-macros/src/validations/custom.rs
new file mode 100644
index 0000000..aeeb779
--- /dev/null
+++ b/packages/fortifier-macros/src/validations/custom.rs
@@ -0,0 +1,77 @@
+use proc_macro2::TokenStream;
+use quote::{ToTokens, quote};
+use syn::{LitBool, Path, Result, Type, meta::ParseNestedMeta};
+
+use crate::validation::Validation;
+
+pub struct Custom {
+    is_async: bool,
+    error_type: Type,
+    function_path: Path,
+}
+
+impl Validation for Custom {
+    fn parse(meta: &ParseNestedMeta<'_>) -> Result<Self> {
+        let mut is_async = false;
+        let mut error_type: Option<Type> = None;
+        let mut function_path: Option<Path> = None;
+
+        meta.parse_nested_meta(|meta| {
+            if meta.path.is_ident("async") {
+                if let Ok(value) = meta.value() {
+                    let lit: LitBool = value.parse()?;
+                    is_async = lit.value;
+                } else {
+                    is_async = true;
+                }
+
+                Ok(())
+            } else if meta.path.is_ident("error") {
+                error_type = Some(meta.value()?.parse()?);
+
+                Ok(())
+            } else if meta.path.is_ident("function") {
+                function_path = Some(meta.value()?.parse()?);
+
+                Ok(())
+            } else {
+                Err(meta.error("unknown parameter"))
+            }
+        })?;
+
+        let Some(error_type) = error_type else {
+            return Err(meta.error("missing error parameter"));
+        };
+        let Some(function_path) = function_path else {
+            return Err(meta.error("missing function parameter"));
+        };
+
+        Ok(Custom {
+            is_async,
+            error_type,
+            function_path,
+        })
+    }
+
+    fn is_async(&self) -> bool {
+        self.is_async
+    }
+
+    fn error_type(&self) -> TokenStream {
+        self.error_type.to_token_stream()
+    }
+
+    fn tokens(&self, expr: &TokenStream) -> TokenStream {
+        let function_path = &self.function_path;
+
+        if self.is_async {
+            quote! {
+                #function_path(&#expr).await
+            }
+        } else {
+            quote! {
+                #function_path(&#expr)
+            }
+        }
+    }
+}