diff --git a/src/poetry/console/commands/update.py b/src/poetry/console/commands/update.py
index b749b01ab6c..d1dec7e80fc 100644
--- a/src/poetry/console/commands/update.py
+++ b/src/poetry/console/commands/update.py
@@ -1,10 +1,13 @@
 from __future__ import annotations
 
+import re
+
 from typing import TYPE_CHECKING
 from typing import ClassVar
 
 from cleo.helpers import argument
 from cleo.helpers import option
+from packaging.utils import canonicalize_name
 
 from poetry.console.commands.installer_command import InstallerCommand
 
@@ -13,6 +16,8 @@
     from cleo.io.inputs.argument import Argument
     from cleo.io.inputs.option import Option
 
+_VERSION_SPECIFIER_RE = re.compile(r"[><=!~]")
+
 
 class UpdateCommand(InstallerCommand):
     name = "update"
@@ -45,6 +50,39 @@ class UpdateCommand(InstallerCommand):
     def handle(self) -> int:
         packages = self.argument("packages")
         if packages:
+            # Detect version specifiers in package arguments — poetry update
+            # only accepts bare package names, not requirement strings.
+            packages_with_specifiers = [
+                p for p in packages if _VERSION_SPECIFIER_RE.search(p)
+            ]
+            if packages_with_specifiers:
+                self.line_error(
+                    "<error>Version specifiers are not allowed in"
+                    " <c1>poetry update</c1>.</error>"
+                )
+                for pkg in packages_with_specifiers:
+                    self.line_error(f"  - {pkg}")
+                self.line_error(
+                    "Use <c1>poetry add</c1> to change version constraints."
+                )
+                return 1
+
+            # Validate that all specified packages are declared dependencies
+            all_dependencies = {
+                canonicalize_name(dep.name) for dep in self.poetry.package.all_requires
+            }
+
+            invalid_packages = [
+                p for p in packages if canonicalize_name(p) not in all_dependencies
+            ]
+
+            if invalid_packages:
+                self.line_error(
+                    "<error>The following packages are not dependencies"
+                    f" of this project: {', '.join(invalid_packages)}</error>"
+                )
+                return 1
+
             self.installer.whitelist(dict.fromkeys(packages, "*"))
 
         self.installer.only_groups(self.activated_groups)
diff --git a/tests/console/commands/test_update.py b/tests/console/commands/test_update.py
index 6b48071d612..3f8da1d87ee 100644
--- a/tests/console/commands/test_update.py
+++ b/tests/console/commands/test_update.py
@@ -100,3 +100,72 @@ def test_update_sync_option_is_passed_to_the_installer(
     tester.execute("--sync")
 
     assert tester.command.installer._requires_synchronization
+
+
+def test_update_with_invalid_package_name_shows_error(
+    poetry_with_outdated_lockfile: Poetry,
+    command_tester_factory: CommandTesterFactory,
+) -> None:
+    """
+    Providing non-existent package names should raise an error.
+    """
+    tester = command_tester_factory("update", poetry=poetry_with_outdated_lockfile)
+
+    status = tester.execute("nonexistent-package")
+
+    assert status == 1
+    assert (
+        "The following packages are not dependencies of this project: nonexistent-package"
+        in tester.io.fetch_error()
+    )
+
+
+def test_update_with_multiple_invalid_package_names_shows_error(
+    poetry_with_outdated_lockfile: Poetry,
+    command_tester_factory: CommandTesterFactory,
+) -> None:
+    """
+    Providing multiple non-existent package names should list all of them in the error.
+    """
+    tester = command_tester_factory("update", poetry=poetry_with_outdated_lockfile)
+
+    status = tester.execute("fake1 fake2 fake3")
+
+    assert status == 1
+    error = tester.io.fetch_error()
+    assert "The following packages are not dependencies of this project" in error
+    assert "fake1" in error
+    assert "fake2" in error
+    assert "fake3" in error
+
+
+@pytest.mark.parametrize(
+    "package_spec",
+    [
+        "docker==1.2.3",
+        "docker>=1.0,<2.0",
+        "docker!=1.0",
+        "docker[extra]>=1.0",
+        "nonexistent==1.2.3",
+        "nonexistent>=1.0",
+    ],
+)
+def test_update_with_version_specifier_raises_error(
+    package_spec: str,
+    poetry_with_outdated_lockfile: Poetry,
+    command_tester_factory: CommandTesterFactory,
+) -> None:
+    """
+    The update command only accepts bare package names. Passing requirement
+    strings with version specifiers should raise a clear error pointing
+    to poetry add, regardless of whether the package is a dependency.
+    """
+    tester = command_tester_factory("update", poetry=poetry_with_outdated_lockfile)
+
+    status = tester.execute(package_spec)
+
+    assert status == 1
+    error = tester.io.fetch_error()
+    assert "Version specifiers are not allowed" in error
+    assert "poetry update" in error
+    assert "poetry add" in error