diff --git a/res/css/_components.scss b/res/css/_components.scss
index 1f896d270d..8bea138acb 100644
--- a/res/css/_components.scss
+++ b/res/css/_components.scss
@@ -100,6 +100,7 @@
 @import "./views/elements/_ToggleSwitch.scss";
 @import "./views/elements/_ToolTipButton.scss";
 @import "./views/elements/_Tooltip.scss";
+@import "./views/elements/_Validation.scss";
 @import "./views/globals/_MatrixToolbar.scss";
 @import "./views/groups/_GroupPublicityToggle.scss";
 @import "./views/groups/_GroupRoomList.scss";
diff --git a/res/css/views/elements/_Validation.scss b/res/css/views/elements/_Validation.scss
new file mode 100644
index 0000000000..08ae793663
--- /dev/null
+++ b/res/css/views/elements/_Validation.scss
@@ -0,0 +1,32 @@
+/*
+Copyright 2019 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+.mx_Validation_details {
+    padding-left: 15px;
+}
+
+.mx_Validation_detail {
+    font-weight: normal;
+
+    // TODO: Check / cross images
+    &.mx_Validation_valid {
+        color: $input-valid-border-color;
+    }
+
+    &.mx_Validation_invalid {
+        color: $input-invalid-border-color;
+    }
+}
diff --git a/src/components/views/auth/RegistrationForm.js b/src/components/views/auth/RegistrationForm.js
index 1784ab61c3..83ea4dfae6 100644
--- a/src/components/views/auth/RegistrationForm.js
+++ b/src/components/views/auth/RegistrationForm.js
@@ -25,6 +25,7 @@ import Modal from '../../../Modal';
 import { _t } from '../../../languageHandler';
 import SdkConfig from '../../../SdkConfig';
 import { SAFE_LOCALPART_REGEX } from '../../../Registration';
+import withValidation from '../elements/Validation';
 
 const FIELD_EMAIL = 'field_email';
 const FIELD_PHONE_NUMBER = 'field_phone_number';
@@ -86,6 +87,7 @@ module.exports = React.createClass({
         // is the one from the first invalid field.
         // It's not super ideal that this just calls
         // onValidationChange once for each invalid field.
+        // TODO: Change this to trigger new-style validation for an invalid fields.
         this.validateField(FIELD_PHONE_NUMBER, ev.type);
         this.validateField(FIELD_EMAIL, ev.type);
         this.validateField(FIELD_PASSWORD_CONFIRM, ev.type);
@@ -152,6 +154,8 @@ module.exports = React.createClass({
         const pwd2 = this.state.passwordConfirm.trim();
         const allowEmpty = eventType === "blur";
 
+        // TODO: Remove rules here as they are converted to new-style validation
+
         switch (fieldID) {
             case FIELD_EMAIL: {
                 const email = this.state.email;
@@ -173,12 +177,6 @@ module.exports = React.createClass({
                 const username = this.state.username;
                 if (allowEmpty && username === '') {
                     this.markFieldValid(fieldID, true);
-                } else if (!SAFE_LOCALPART_REGEX.test(username)) {
-                    this.markFieldValid(
-                        fieldID,
-                        false,
-                        "RegistrationForm.ERR_USERNAME_INVALID",
-                    );
                 } else if (username == '') {
                     this.markFieldValid(
                         fieldID,
@@ -232,11 +230,14 @@ module.exports = React.createClass({
         this.setState({
             fieldErrors,
         });
+        // TODO: Remove outer validation handling once all fields converted to new-style
+        // validation in the form.
         this.props.onValidationChange(fieldErrors);
     },
 
     _classForField: function(fieldID, ...baseClasses) {
         let cls = baseClasses.join(' ');
+        // TODO: Remove this from fields as they are converted to new-style validation.
         if (this.state.fieldErrors[fieldID]) {
             if (cls) cls += ' ';
             cls += 'error';
@@ -291,10 +292,6 @@ module.exports = React.createClass({
         });
     },
 
-    onUsernameBlur(ev) {
-        this.validateField(FIELD_USERNAME, ev.type);
-    },
-
     onUsernameChange(ev) {
         this.setState({
             username: ev.target.value,
@@ -325,6 +322,33 @@ module.exports = React.createClass({
         });
     },
 
+    renderUsername() {
+        const Field = sdk.getComponent('elements.Field');
+
+        const onValidate = withValidation({
+            description: _t("Use letters, numbers, dashes and underscores only"),
+            rules: [
+                {
+                    key: "safeLocalpart",
+                    regex: SAFE_LOCALPART_REGEX,
+                    invalid: _t("Some characters not allowed"),
+                },
+            ],
+        });
+
+        return <Field
+            className={this._classForField(FIELD_USERNAME)}
+            id="mx_RegistrationForm_username"
+            type="text"
+            autoFocus={true}
+            label={_t("Username")}
+            defaultValue={this.props.defaultUsername}
+            value={this.state.username}
+            onChange={this.onUsernameChange}
+            onValidate={onValidate}
+        />;
+    },
+
     render: function() {
         const Field = sdk.getComponent('elements.Field');
 
@@ -412,17 +436,7 @@ module.exports = React.createClass({
                 </h3>
                 <form onSubmit={this.onSubmit}>
                     <div className="mx_AuthBody_fieldRow">
-                        <Field
-                            className={this._classForField(FIELD_USERNAME)}
-                            id="mx_RegistrationForm_username"
-                            type="text"
-                            autoFocus={true}
-                            label={_t("Username")}
-                            defaultValue={this.props.defaultUsername}
-                            value={this.state.username}
-                            onBlur={this.onUsernameBlur}
-                            onChange={this.onUsernameChange}
-                        />
+                        {this.renderUsername()}
                     </div>
                     <div className="mx_AuthBody_fieldRow">
                         <Field
diff --git a/src/components/views/elements/Field.js b/src/components/views/elements/Field.js
index 1b9d55c221..6182a80b70 100644
--- a/src/components/views/elements/Field.js
+++ b/src/components/views/elements/Field.js
@@ -94,11 +94,11 @@ export default class Field extends React.PureComponent {
             mx_Field_invalid: onValidate && this.state.valid === false,
         });
 
-        // handle displaying feedback on validity
+        // Handle displaying feedback on validity
         const Tooltip = sdk.getComponent("elements.Tooltip");
-        let feedback;
+        let tooltip;
         if (this.state.feedback) {
-            feedback = <Tooltip
+            tooltip = <Tooltip
                 tooltipClassName="mx_Field_tooltip"
                 label={this.state.feedback}
             />;
@@ -108,7 +108,7 @@ export default class Field extends React.PureComponent {
             {prefixContainer}
             {fieldInput}
             <label htmlFor={this.props.id}>{this.props.label}</label>
-            {feedback}
+            {tooltip}
         </div>;
     }
 }
diff --git a/src/components/views/elements/Validation.js b/src/components/views/elements/Validation.js
new file mode 100644
index 0000000000..21538609c1
--- /dev/null
+++ b/src/components/views/elements/Validation.js
@@ -0,0 +1,102 @@
+/*
+Copyright 2019 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import classNames from 'classnames';
+
+/**
+ * Creates a validation function from a set of rules describing what to validate.
+ *
+ * @param {String} description
+ *     Summary of the kind of value that will meet the validation rules. Shown at
+ *     the top of the validation feedback.
+ * @param {Object} rules
+ *     An array of rules describing how to check to input value. Each rule in an object
+ *     and may have the following properties:
+ *     - `key`: A unique ID for the rule. Required.
+ *     - `regex`: A regex used to determine the rule's current validity. Required.
+ *     - `valid`: Text to show when the rule is valid. Only shown if set.
+ *     - `invalid`: Text to show when the rule is invalid. Only shown if set.
+ * @returns {Function}
+ *     A validation function that takes in the current input value and returns
+ *     the overall validity and a feedback UI that can be rendered for more detail.
+ */
+export default function withValidation({ description, rules }) {
+    return function onValidate(value) {
+        // TODO: Hide on blur
+        // TODO: Re-run only after ~200ms of inactivity
+        if (!value) {
+            return {
+                valid: null,
+                feedback: null,
+            };
+        }
+
+        const results = [];
+        let valid = true;
+        if (rules && rules.length) {
+            for (const rule of rules) {
+                if (!rule.key || !rule.regex) {
+                    continue;
+                }
+                const ruleValid = rule.regex.test(value);
+                valid = valid && ruleValid;
+                if (ruleValid && rule.valid) {
+                    // If the rule's result is valid and has text to show for
+                    // the valid state, show it.
+                    results.push({
+                        key: rule.key,
+                        valid: true,
+                        text: rule.valid,
+                    });
+                } else if (!ruleValid && rule.invalid) {
+                    // If the rule's result is invalid and has text to show for
+                    // the invalid state, show it.
+                    results.push({
+                        key: rule.key,
+                        valid: false,
+                        text: rule.invalid,
+                    });
+                }
+            }
+        }
+
+        let details;
+        if (results && results.length) {
+            details = <ul className="mx_Validation_details">
+                {results.map(result => {
+                    const classes = classNames({
+                        "mx_Validation_detail": true,
+                        "mx_Validation_valid": result.valid,
+                        "mx_Validation_invalid": !result.valid,
+                    });
+                    return <li key={result.key} className={classes}>
+                        {result.text}
+                    </li>;
+                })}
+            </ul>;
+        }
+
+        const feedback = <div className="mx_Validation">
+            <div>{description}</div>
+            {details}
+        </div>;
+
+        return {
+            valid,
+            feedback,
+        };
+    };
+}
diff --git a/src/i18n/strings/en_EN.json b/src/i18n/strings/en_EN.json
index a7fae2803f..51967c39f1 100644
--- a/src/i18n/strings/en_EN.json
+++ b/src/i18n/strings/en_EN.json
@@ -1322,6 +1322,8 @@
     "Change": "Change",
     "Sign in with": "Sign in with",
     "If you don't specify an email address, you won't be able to reset your password. Are you sure?": "If you don't specify an email address, you won't be able to reset your password. Are you sure?",
+    "Use letters, numbers, dashes and underscores only": "Use letters, numbers, dashes and underscores only",
+    "Some characters not allowed": "Some characters not allowed",
     "Create your Matrix account": "Create your Matrix account",
     "Create your Matrix account on %(serverName)s": "Create your Matrix account on %(serverName)s",
     "Email (optional)": "Email (optional)",