diff --git a/packages/form-js-viewer/src/core/Validator.js b/packages/form-js-viewer/src/core/Validator.js
index ab197f474..b614998a6 100644
--- a/packages/form-js-viewer/src/core/Validator.js
+++ b/packages/form-js-viewer/src/core/Validator.js
@@ -1,11 +1,14 @@
 import { isNil, get, set } from 'min-dash';
 import { countDecimals } from '../render/components/util/numberFieldUtil';
 import Big from 'big.js';
+import { clone } from '../util';
+import { wrapObjectKeysWithUnderscores } from '../util/simple';
 
 const EMAIL_PATTERN = /^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$/;
 const PHONE_PATTERN = /(\+|00)(297|93|244|1264|358|355|376|971|54|374|1684|1268|61|43|994|257|32|229|226|880|359|973|1242|387|590|375|501|1441|591|55|1246|673|975|267|236|1|61|41|56|86|225|237|243|242|682|57|269|238|506|53|5999|61|1345|357|420|49|253|1767|45|1809|1829|1849|213|593|20|291|212|34|372|251|358|679|500|33|298|691|241|44|995|44|233|350|224|590|220|245|240|30|1473|299|502|594|1671|592|852|504|385|509|36|62|44|91|246|353|98|964|354|972|39|1876|44|962|81|76|77|254|996|855|686|1869|82|383|965|856|961|231|218|1758|423|94|266|370|352|371|853|590|212|377|373|261|960|52|692|389|223|356|95|382|976|1670|258|222|1664|596|230|265|60|262|264|687|227|672|234|505|683|31|47|977|674|64|968|92|507|64|51|63|680|675|48|1787|1939|850|351|595|970|689|974|262|40|7|250|966|249|221|65|500|4779|677|232|503|378|252|508|381|211|239|597|421|386|46|268|1721|248|963|1649|235|228|66|992|690|993|670|676|1868|216|90|688|886|255|256|380|598|1|998|3906698|379|1784|58|1284|1340|84|678|681|685|967|27|260|263)(9[976]\d|8[987530]\d|6[987]\d|5[90]\d|42\d|3[875]\d|2[98654321]\d|9[8543210]|8[6421]|6[6543210]|5[87654321]|4[987654310]|3[9643210]|2[70]|7|1)\d{4,20}$/;
 
-const VALIDATE_FEEL_PROPERTIES = [
+const PRECOMPUTE_FEEL_PROPERTIES = [
+  'custom',
   'min',
   'max',
   'minLength',
@@ -75,7 +78,7 @@ export class Validator {
       return errors;
     }
 
-    const evaluatedValidation = evaluateFEELValues(
+    const evaluatedValidation = precomputeFEELValues(
       validate,
       this._expressionLanguage,
       this._conditionChecker,
@@ -144,6 +147,20 @@ export class Validator {
       ];
     }
 
+    if ('custom' in evaluatedValidation && value && evaluatedValidation.custom.length) {
+      const { custom } = evaluatedValidation;
+      custom.forEach(customValidation => {
+        const { condition, message } = customValidation;
+
+        if (condition && !evaluateExpression(condition, this._expressionLanguage, this._conditionChecker, this._form, { value })) {
+          errors = [
+            ...errors,
+            message
+          ];
+        }
+      });
+    }
+
     return errors;
   }
 }
@@ -153,35 +170,29 @@ Validator.$inject = [ 'expressionLanguage', 'conditionChecker', 'form' ];
 
 // helpers //////////
 
-/**
- * Helper function to evaluate optional FEEL validation values.
- */
-function evaluateFEELValues(validate, expressionLanguage, conditionChecker, form) {
-
-  const evaluatedValidate = { ...validate };
-
-  VALIDATE_FEEL_PROPERTIES.forEach(property => {
-    const path = property.split('.');
+function precomputeFEELValues(_validate, expressionLanguage, conditionChecker, form) {
 
-    const value = get(evaluatedValidate, path);
+  const validate = clone(_validate);
 
-    // mirroring FEEL evaluation of our hooks
-    if (!expressionLanguage || !expressionLanguage.isExpression(value)) {
-      return value;
+  PRECOMPUTE_FEEL_PROPERTIES.forEach(propertyPath => {
+    const value = get(validate, propertyPath.split('.'));
+    const evaluatedValue = evaluateExpression(value, expressionLanguage, conditionChecker, form);
+    if (evaluatedValue) {
+      set(validate, propertyPath.split('.'), evaluatedValue);
     }
+  });
 
-    const { initialData, data } = form._getState();
-
-    const newData = conditionChecker ? conditionChecker.applyConditions(data, data) : data;
-    const filteredData = { ...initialData, ...newData };
+  return validate;
+}
 
-    const evaluatedValue = expressionLanguage.evaluate(value, filteredData);
+function evaluateExpression(expression, expressionLanguage, conditionChecker, form, additionalContext = {}) {
+  if (!expressionLanguage || !expressionLanguage.isExpression(expression)) {
+    return expression;
+  }
 
-    // replace validate property with evaluated value
-    if (evaluatedValue) {
-      set(evaluatedValidate, path, evaluatedValue);
-    }
-  });
+  const { initialData, data } = form._getState();
+  const filteredData = conditionChecker ? conditionChecker.applyConditions(data, data) : data;
+  const expressionContext = { ...additionalContext, ...initialData, ...filteredData, ...wrapObjectKeysWithUnderscores(additionalContext) };
 
-  return evaluatedValidate;
+  return expressionLanguage.evaluate(expression, expressionContext);
 }
\ No newline at end of file
diff --git a/packages/form-js-viewer/src/util/simple.js b/packages/form-js-viewer/src/util/simple.js
index 6cf8665cc..3e6f40872 100644
--- a/packages/form-js-viewer/src/util/simple.js
+++ b/packages/form-js-viewer/src/util/simple.js
@@ -63,7 +63,7 @@ export function buildExpressionContext(context) {
   return {
     ...specialContextKeys,
     ...data,
-    ..._wrapObjectKeysWithUnderscores(specialContextKeys)
+    ...wrapObjectKeysWithUnderscores(specialContextKeys)
   };
 }
 
@@ -77,9 +77,7 @@ export function runRecursively(formField, fn) {
   fn(formField);
 }
 
-// helpers //////////////////////
-
-function _wrapObjectKeysWithUnderscores(obj) {
+export function wrapObjectKeysWithUnderscores(obj) {
   const newObj = {};
   for (const [ key, value ] of Object.entries(obj)) {
     newObj[`_${key}_`] = value;