diff --git a/moto/organizations/exceptions.py b/moto/organizations/exceptions.py
index 01b98da7e..b40908862 100644
--- a/moto/organizations/exceptions.py
+++ b/moto/organizations/exceptions.py
@@ -10,3 +10,13 @@ class InvalidInputException(JsonRESTError):
             "InvalidInputException",
             "You provided a value that does not match the required pattern.",
         )
+
+
+class DuplicateOrganizationalUnitException(JsonRESTError):
+    code = 400
+
+    def __init__(self):
+        super(DuplicateOrganizationalUnitException, self).__init__(
+            "DuplicateOrganizationalUnitException",
+            "An OU with the same name already exists.",
+        )
diff --git a/moto/organizations/models.py b/moto/organizations/models.py
index 9be129fa7..0db069f9a 100644
--- a/moto/organizations/models.py
+++ b/moto/organizations/models.py
@@ -8,7 +8,10 @@ from moto.core import BaseBackend, BaseModel
 from moto.core.exceptions import RESTError
 from moto.core.utils import unix_time
 from moto.organizations import utils
-from moto.organizations.exceptions import InvalidInputException
+from moto.organizations.exceptions import (
+    InvalidInputException,
+    DuplicateOrganizationalUnitException,
+)
 
 
 class FakeOrganization(BaseModel):
@@ -223,6 +226,9 @@ class OrganizationsBackend(BaseBackend):
         return new_ou.describe()
 
     def update_organizational_unit(self, **kwargs):
+        for ou in self.ou:
+            if ou.name == kwargs["Name"]:
+                raise DuplicateOrganizationalUnitException
         ou = self.get_organizational_unit_by_id(kwargs["OrganizationalUnitId"])
         ou.name = kwargs["Name"]
         return ou.describe()
diff --git a/tests/test_organizations/test_organizations_boto3.py b/tests/test_organizations/test_organizations_boto3.py
index ab3ddf671..876e83712 100644
--- a/tests/test_organizations/test_organizations_boto3.py
+++ b/tests/test_organizations/test_organizations_boto3.py
@@ -730,3 +730,24 @@ def test_update_organizational_unit():
     )
     validate_organizational_unit(org, response)
     response["OrganizationalUnit"]["Name"].should.equal(new_ou_name)
+
+
+@mock_organizations
+def test_update_organizational_unit_duplicate_error():
+    client = boto3.client("organizations", region_name="us-east-1")
+    org = client.create_organization(FeatureSet="ALL")["Organization"]
+    root_id = client.list_roots()["Roots"][0]["Id"]
+    ou_name = "ou01"
+    response = client.create_organizational_unit(ParentId=root_id, Name=ou_name)
+    validate_organizational_unit(org, response)
+    response["OrganizationalUnit"]["Name"].should.equal(ou_name)
+    with assert_raises(ClientError) as e:
+        client.update_organizational_unit(
+            OrganizationalUnitId=response["OrganizationalUnit"]["Id"], Name=ou_name
+        )
+    exc = e.exception
+    exc.operation_name.should.equal("UpdateOrganizationalUnit")
+    exc.response["Error"]["Code"].should.contain("DuplicateOrganizationalUnitException")
+    exc.response["Error"]["Message"].should.equal(
+        "An OU with the same name already exists."
+    )