diff --git a/src/ert/gui/simulation/evaluate_ensemble_panel.py b/src/ert/gui/simulation/evaluate_ensemble_panel.py
index 44d7400c4ef..a81b40ad15c 100644
--- a/src/ert/gui/simulation/evaluate_ensemble_panel.py
+++ b/src/ert/gui/simulation/evaluate_ensemble_panel.py
@@ -14,7 +14,7 @@
 from ert.gui.simulation.experiment_config_panel import ExperimentConfigPanel
 from ert.mode_definitions import EVALUATE_ENSEMBLE_MODE
 from ert.run_models.evaluate_ensemble import EvaluateEnsemble
-from ert.validation import RangeStringArgument
+from ert.validation import EnsembleRealizationsArgument
 
 
 @dataclass
@@ -47,9 +47,10 @@ def __init__(self, ensemble_size: int, run_path: str, notifier: ErtNotifier):
             ActiveRealizationsModel(ensemble_size, show_default=False),  # type: ignore
             "config/simulation/active_realizations",
         )
-        self._active_realizations_field.setValidator(
-            RangeStringArgument(ensemble_size),
+        self._realizations_validator = EnsembleRealizationsArgument(
+            self._ensemble_selector.selected_ensemble, max_value=ensemble_size
         )
+        self._active_realizations_field.setValidator(self._realizations_validator)
         self._realizations_from_fs()
         layout.addRow("Active realizations", self._active_realizations_field)
 
@@ -68,7 +69,7 @@ def isConfigurationValid(self) -> bool:
         return (
             self._active_realizations_field.isValid()
             and self._ensemble_selector.currentIndex() != -1
-            and bool(self._active_realizations_field.text())
+            and self._active_realizations_field.isValid()
         )
 
     def get_experiment_arguments(self) -> Arguments:
@@ -80,7 +81,9 @@ def get_experiment_arguments(self) -> Arguments:
 
     def _realizations_from_fs(self) -> None:
         ensemble = self._ensemble_selector.selected_ensemble
+        self._active_realizations_field.setEnabled(ensemble is not None)
         if ensemble:
+            self._realizations_validator.set_ensemble(ensemble)
             parameters = ensemble.get_realization_mask_with_parameters()
             missing_responses = ~ensemble.get_realization_mask_with_responses()
             failures = ~ensemble.get_realization_mask_without_failure()
diff --git a/src/ert/gui/simulation/manual_update_panel.py b/src/ert/gui/simulation/manual_update_panel.py
index b96f75db15f..4a7d2a93be5 100644
--- a/src/ert/gui/simulation/manual_update_panel.py
+++ b/src/ert/gui/simulation/manual_update_panel.py
@@ -17,7 +17,7 @@
 from ert.gui.simulation.experiment_config_panel import ExperimentConfigPanel
 from ert.mode_definitions import MANUAL_UPDATE_MODE
 from ert.run_models.manual_update import ManualUpdate
-from ert.validation import ProperNameFormatArgument, RangeStringArgument
+from ert.validation import EnsembleRealizationsArgument, ProperNameFormatArgument
 
 
 @dataclass
@@ -72,14 +72,13 @@ def __init__(
             ActiveRealizationsModel(ensemble_size, show_default=False),  # type: ignore
             "config/simulation/active_realizations",
         )
-        self._active_realizations_field.setValidator(
-            RangeStringArgument(ensemble_size),
+        self._realizations_validator = EnsembleRealizationsArgument(
+            self._ensemble_selector.selected_ensemble, max_value=ensemble_size
         )
+        self._active_realizations_field.setValidator(self._realizations_validator)
         self._realizations_from_fs()
         layout.addRow("Active realizations", self._active_realizations_field)
 
-        self.setLayout(layout)
-
         self._active_realizations_field.getValidationSupport().validationChanged.connect(
             self.simulationConfigurationChanged
         )
@@ -88,12 +87,13 @@ def __init__(
             self.simulationConfigurationChanged
         )
         self._ensemble_selector.currentIndexChanged.connect(self._realizations_from_fs)
+        self.setLayout(layout)
 
     def isConfigurationValid(self) -> bool:
         return (
             self._active_realizations_field.isValid()
             and self._ensemble_selector.currentIndex() != -1
-            and bool(self._active_realizations_field.text())
+            and self._active_realizations_field.isValid()
         )
 
     def get_experiment_arguments(self) -> Arguments:
@@ -106,7 +106,9 @@ def get_experiment_arguments(self) -> Arguments:
 
     def _realizations_from_fs(self) -> None:
         ensemble = self._ensemble_selector.selected_ensemble
+        self._active_realizations_field.setEnabled(ensemble is not None)
         if ensemble:
+            self._realizations_validator.set_ensemble(ensemble)
             parameters = ensemble.get_realization_mask_with_parameters()
             responses = ensemble.get_realization_mask_with_responses()
             mask = np.logical_and(parameters, responses)
diff --git a/src/ert/validation/__init__.py b/src/ert/validation/__init__.py
index b503bea56cc..7b8ffa7cc01 100644
--- a/src/ert/validation/__init__.py
+++ b/src/ert/validation/__init__.py
@@ -1,5 +1,6 @@
 from .active_range import ActiveRange
 from .argument_definition import ArgumentDefinition
+from .ensemble_realizations_argument import EnsembleRealizationsArgument
 from .integer_argument import IntegerArgument
 from .number_list_string_argument import NumberListStringArgument
 from .proper_name_argument import ProperNameArgument
@@ -12,6 +13,7 @@
 __all__ = [
     "ActiveRange",
     "ArgumentDefinition",
+    "EnsembleRealizationsArgument",
     "IntegerArgument",
     "NumberListStringArgument",
     "ProperNameArgument",
diff --git a/src/ert/validation/ensemble_realizations_argument.py b/src/ert/validation/ensemble_realizations_argument.py
new file mode 100644
index 00000000000..4220b00b65f
--- /dev/null
+++ b/src/ert/validation/ensemble_realizations_argument.py
@@ -0,0 +1,49 @@
+from typing import TYPE_CHECKING, Optional
+
+from .range_string_argument import RangeStringArgument
+from .rangestring import rangestring_to_list
+from .validation_status import ValidationStatus
+
+if TYPE_CHECKING:
+    from ert.storage import Ensemble
+
+
+class EnsembleRealizationsArgument(RangeStringArgument):
+    UNINITIALIZED_REALIZATIONS_SPECIFIED = (
+        "The specified realization(s) %s are not found in selected ensemble."
+    )
+
+    def __init__(
+        self, ensemble: "Ensemble", max_value: Optional[int], **kwargs: bool
+    ) -> None:
+        super().__init__(max_value, **kwargs)
+        self.__ensemble = ensemble
+
+    def set_ensemble(self, ensemble: "Ensemble") -> None:
+        self.__ensemble = ensemble
+
+    def validate(self, token: str) -> ValidationStatus:
+        if not token:
+            return ValidationStatus()
+
+        validation_status = super().validate(token)
+        if not validation_status:
+            return validation_status
+        attempted_realizations = rangestring_to_list(token)
+
+        invalid_realizations = []
+        initialized_realization_ids = self.__ensemble.is_initalized()
+        for realization in attempted_realizations:
+            if not realization in initialized_realization_ids:
+                invalid_realizations.append(realization)
+
+        if invalid_realizations:
+            validation_status.setFailed()
+            validation_status.addToMessage(
+                EnsembleRealizationsArgument.UNINITIALIZED_REALIZATIONS_SPECIFIED
+                % str(invalid_realizations)
+            )
+            return validation_status
+
+        validation_status.setValue(token)
+        return validation_status