diff --git a/crates/domain/src/id.rs b/crates/domain/src/id.rs
new file mode 100644
index 0000000..382c249
--- /dev/null
+++ b/crates/domain/src/id.rs
@@ -0,0 +1,66 @@
+use std::fmt;
+use std::str::FromStr;
+
+use serde::{Deserialize, Serialize};
+use uuid::Uuid;
+
+/// Identifier for an organization (tenant).
+///
+/// A newtype over [`Uuid`] so it can never be confused with another entity's id.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
+#[serde(transparent)]
+pub struct OrgId(Uuid);
+
+impl OrgId {
+    /// Generate a fresh random id.
+    pub fn new() -> Self {
+        Self(Uuid::new_v4())
+    }
+
+    /// Wrap an existing [`Uuid`].
+    pub fn from_uuid(uuid: Uuid) -> Self {
+        Self(uuid)
+    }
+
+    /// The underlying [`Uuid`].
+    pub fn as_uuid(&self) -> Uuid {
+        self.0
+    }
+}
+
+impl Default for OrgId {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl fmt::Display for OrgId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+impl FromStr for OrgId {
+    type Err = uuid::Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Ok(Self(Uuid::parse_str(s)?))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parses_and_displays_round_trip() {
+        let text = "550e8400-e29b-41d4-a716-446655440000";
+        let id: OrgId = text.parse().expect("valid uuid should parse");
+        assert_eq!(id.to_string(), text);
+    }
+
+    #[test]
+    fn rejects_invalid_uuid() {
+        assert!("not-a-uuid".parse::<OrgId>().is_err());
+    }
+}
diff --git a/crates/domain/src/lib.rs b/crates/domain/src/lib.rs
index 9c37b95..5972e72 100644
--- a/crates/domain/src/lib.rs
+++ b/crates/domain/src/lib.rs
@@ -1 +1,5 @@
 //! Core domain types and invariants. No I/O dependencies.
+
+mod id;
+
+pub use id::OrgId;