diff --git a/native/account/log-in-panel.react.js b/native/account/log-in-panel.react.js
--- a/native/account/log-in-panel.react.js
+++ b/native/account/log-in-panel.react.js
@@ -180,7 +180,7 @@
   };
 
   onChangeUsernameInputText: (text: string) => void = text => {
-    this.props.logInState.setState({ usernameInputText: text });
+    this.props.logInState.setState({ usernameInputText: text.trim() });
   };
 
   onUsernameKeyPress: (event: KeyPressEvent) => void = event => {
diff --git a/web/account/log-in-form.react.js b/web/account/log-in-form.react.js
--- a/web/account/log-in-form.react.js
+++ b/web/account/log-in-form.react.js
@@ -94,12 +94,12 @@
 
       if (username.search(validEmailRegex) > -1) {
         setUsername('');
-        setErrorMessage('usernames only, not emails');
+        setErrorMessage('Usernames only, not emails');
         usernameInputRef.current?.focus();
         return;
       } else if (username.search(oldValidUsernameRegex) === -1) {
         setUsername('');
-        setErrorMessage('alphanumeric usernames only');
+        setErrorMessage('Alphanumeric usernames only');
         usernameInputRef.current?.focus();
         return;
       }
@@ -133,6 +133,9 @@
               placeholder="Username"
               value={username}
               onChange={onUsernameChange}
+              onBlur={() => {
+                setUsername(untrimmedUsername => untrimmedUsername.trim());
+              }}
               ref={usernameInputRef}
               disabled={inputDisabled}
             />
diff --git a/web/modals/input.react.js b/web/modals/input.react.js
--- a/web/modals/input.react.js
+++ b/web/modals/input.react.js
@@ -9,6 +9,7 @@
   +placeholder: string,
   +value: string,
   +onChange: (value: SyntheticEvent<HTMLInputElement>) => mixed,
+  +onBlur?: (value: SyntheticEvent<HTMLInputElement>) => mixed,
   +disabled?: boolean,
   +label?: string,
   +id?: string,