diff --git a/proxmox-api-macro/src/api/mod.rs b/proxmox-api-macro/src/api/mod.rs
index abd1b036..e346101b 100644
--- a/proxmox-api-macro/src/api/mod.rs
+++ b/proxmox-api-macro/src/api/mod.rs
@@ -357,6 +357,13 @@ impl SchemaItem {
                 ts.extend(quote_spanned! { obj.span =>
                     ::proxmox_schema::ObjectSchema::new(#description, &[#elems])
                 });
+                if obj
+                    .additional_properties
+                    .as_ref()
+                    .is_some_and(|a| a.to_bool())
+                {
+                    ts.extend(quote_spanned! { obj.span => .additional_properties(true) });
+                }
             }
             SchemaItem::Array(array) => {
                 let description = check_description()?;
@@ -516,6 +523,51 @@ impl ObjectEntry {
 pub struct SchemaObject {
     span: Span,
     properties_: Vec<ObjectEntry>,
+    additional_properties: Option<AdditionalProperties>,
+}
+
+#[derive(Clone)]
+pub enum AdditionalProperties {
+    /// `additional_properties: false`.
+    No,
+    /// `additional_properties: true`.
+    Ignored,
+    /// `additional_properties: "field_name"`.
+    Field(syn::LitStr),
+}
+
+impl TryFrom<JSONValue> for AdditionalProperties {
+    type Error = syn::Error;
+
+    fn try_from(value: JSONValue) -> Result<Self, Self::Error> {
+        let span = value.span();
+        if let JSONValue::Expr(syn::Expr::Lit(expr_lit)) = value {
+            match expr_lit.lit {
+                syn::Lit::Str(s) => return Ok(Self::Field(s)),
+                syn::Lit::Bool(b) => {
+                    return Ok(if b.value() { Self::Ignored } else { Self::No });
+                }
+                _ => (),
+            }
+        }
+        bail!(
+            span,
+            "invalid value for additional_properties, expected boolean or field name"
+        );
+    }
+}
+
+impl AdditionalProperties {
+    pub fn to_option_string(&self) -> Option<String> {
+        match self {
+            Self::Field(name) => Some(name.value()),
+            _ => None,
+        }
+    }
+
+    pub fn to_bool(&self) -> bool {
+        !matches!(self, Self::No)
+    }
 }
 
 impl SchemaObject {
@@ -523,6 +575,7 @@ impl SchemaObject {
         Self {
             span,
             properties_: Vec::new(),
+            additional_properties: None,
         }
     }
 
@@ -574,6 +627,10 @@ impl SchemaObject {
     fn try_extract_from(obj: &mut JSONObject) -> Result<Self, syn::Error> {
         let mut this = Self {
             span: obj.span(),
+            additional_properties: obj
+                .remove("additional_properties")
+                .map(AdditionalProperties::try_from)
+                .transpose()?,
             properties_: obj
                 .remove_required_element("properties")?
                 .into_object("object field definition")?
diff --git a/proxmox-api-macro/src/api/structs.rs b/proxmox-api-macro/src/api/structs.rs
index aac74817..ee537ff0 100644
--- a/proxmox-api-macro/src/api/structs.rs
+++ b/proxmox-api-macro/src/api/structs.rs
@@ -141,9 +141,15 @@ fn handle_regular_struct(
     // fields if there are any.
     let mut schema_fields: HashMap<String, &mut ObjectEntry> = HashMap::new();
 
+    let mut additional_properties = None;
+
     // We also keep a reference to the SchemaObject around since we derive missing fields
     // automatically.
     if let SchemaItem::Object(obj) = &mut schema.item {
+        additional_properties = obj
+            .additional_properties
+            .as_ref()
+            .and_then(|a| a.to_option_string());
         for field in obj.properties_mut() {
             schema_fields.insert(field.name.as_str().to_string(), field);
         }
@@ -178,6 +184,12 @@ fn handle_regular_struct(
                 }
             };
 
+            if additional_properties.as_deref() == Some(name.as_ref()) {
+                // we just *skip* the additional properties field, it is supposed to be a flattened
+                // `HashMap<String, Value>` collecting all the values that have no schema
+                continue;
+            }
+
             match schema_fields.remove(&name) {
                 Some(field_def) => {
                     if attrs.flatten {
diff --git a/proxmox-api-macro/tests/types.rs b/proxmox-api-macro/tests/types.rs
index b4b7250e..e645fa84 100644
--- a/proxmox-api-macro/tests/types.rs
+++ b/proxmox-api-macro/tests/types.rs
@@ -3,6 +3,8 @@
 
 #![allow(dead_code)]
 
+use std::collections::HashMap;
+
 use proxmox_api_macro::api;
 use proxmox_schema as schema;
 use proxmox_schema::{ApiType, EnumEntry};
@@ -11,6 +13,8 @@ use anyhow::Error;
 use serde::Deserialize;
 use serde_json::Value;
 
+pub const TEXT_SCHEMA: schema::Schema = schema::StringSchema::new("Text.").schema();
+
 #[api(
     type: String,
     description: "A string",
@@ -186,3 +190,27 @@ fn string_check_schema_test() {
 pub struct RenamedAndDescribed {
     a_field: String,
 }
+
+#[api(
+    properties: {},
+    additional_properties: "rest",
+)]
+#[derive(Deserialize)]
+/// Some Description.
+pub struct UnspecifiedData {
+    /// Text.
+    field: String,
+
+    /// Remaining data.
+    rest: HashMap<String, Value>,
+}
+
+#[test]
+fn additional_properties_test() {
+    const TEST_UNSPECIFIED: ::proxmox_schema::Schema =
+        ::proxmox_schema::ObjectSchema::new("Some Description.", &[("field", false, &TEXT_SCHEMA)])
+            .additional_properties(true)
+            .schema();
+
+    assert_eq!(TEST_UNSPECIFIED, UnspecifiedData::API_SCHEMA);
+}