diff --git a/securedrop/models.py b/securedrop/models.py
index 402c979ed1..38e07a9e47 100644
--- a/securedrop/models.py
+++ b/securedrop/models.py
@@ -184,6 +184,8 @@ def to_json(self):
 
 
 class Submission(db.Model):
+    MAX_MESSAGE_LEN = 100000
+
     __tablename__ = 'submissions'
     id = Column(Integer, primary_key=True)
     uuid = Column(String(36), unique=True, nullable=False)
diff --git a/securedrop/source_app/forms.py b/securedrop/source_app/forms.py
index 353de62af4..715fbddb40 100644
--- a/securedrop/source_app/forms.py
+++ b/securedrop/source_app/forms.py
@@ -1,9 +1,10 @@
+from flask import current_app
 from flask_babel import lazy_gettext as gettext
 from flask_wtf import FlaskForm
-from wtforms import PasswordField
-from wtforms.validators import InputRequired, Regexp, Length
+from wtforms import FileField, PasswordField, TextAreaField
+from wtforms.validators import InputRequired, Regexp, Length, ValidationError
 
-from models import Source
+from models import Source, Submission
 
 
 class LoginForm(FlaskForm):
@@ -17,3 +18,20 @@ class LoginForm(FlaskForm):
         # Make sure to allow dashes since some words in the wordlist have them
         Regexp(r'[\sA-Za-z0-9-]+$', message=gettext('Invalid input.'))
     ])
+
+
+class SubmissionForm(FlaskForm):
+    msg = TextAreaField("msg", render_kw={"placeholder": gettext("Write a message.")})
+    fh = FileField("fh")
+
+    def validate_msg(self, field):
+        if len(field.data) > Submission.MAX_MESSAGE_LEN:
+            message = gettext("Message text too long.")
+            if current_app.instance_config.allow_document_uploads:
+                message = "{} {}".format(
+                    message,
+                    gettext(
+                        "Large blocks of text must be uploaded as a file, not copied and pasted."
+                    )
+                )
+            raise ValidationError(message)
diff --git a/securedrop/source_app/main.py b/securedrop/source_app/main.py
index 3b787c56b3..172849fbc6 100644
--- a/securedrop/source_app/main.py
+++ b/securedrop/source_app/main.py
@@ -17,7 +17,7 @@
 from source_app.utils import (logged_in, generate_unique_codename,
                               async_genkey, normalize_timestamps,
                               valid_codename, get_entropy_estimate)
-from source_app.forms import LoginForm
+from source_app.forms import LoginForm, SubmissionForm
 
 
 def make_blueprint(config):
@@ -145,13 +145,21 @@ def lookup():
             replies=replies,
             flagged=g.source.flagged,
             new_user=session.get('new_user', None),
-            haskey=current_app.crypto_util.get_fingerprint(
-                g.filesystem_id))
+            haskey=current_app.crypto_util.get_fingerprint(g.filesystem_id),
+            form=SubmissionForm(),
+        )
 
     @view.route('/submit', methods=('POST',))
     @login_required
     def submit():
         allow_document_uploads = current_app.instance_config.allow_document_uploads
+        form = SubmissionForm()
+        if not form.validate():
+            for field, errors in form.errors.items():
+                for error in errors:
+                    flash(error, "error")
+            return redirect(url_for('main.lookup'))
+
         msg = request.form['msg']
         fh = None
         if allow_document_uploads and 'fh' in request.files:
@@ -190,21 +198,23 @@ def submit():
                     fh.stream))
 
         if first_submission:
-            msg = render_template('first_submission_flashed_message.html')
-            flash(Markup(msg), "success")
+            flash_message = render_template('first_submission_flashed_message.html')
+            flash(Markup(flash_message), "success")
 
         else:
             if msg and not fh:
                 html_contents = gettext('Thanks! We received your message.')
-            elif not msg and fh:
+            elif fh and not msg:
                 html_contents = gettext('Thanks! We received your document.')
             else:
                 html_contents = gettext('Thanks! We received your message and '
                                         'document.')
 
-            msg = render_template('next_submission_flashed_message.html',
-                                  html_contents=html_contents)
-            flash(Markup(msg), "success")
+            flash_message = render_template(
+                'next_submission_flashed_message.html',
+                html_contents=html_contents
+            )
+            flash(Markup(flash_message), "success")
 
         new_submissions = []
         for fname in fnames:
diff --git a/securedrop/source_templates/lookup.html b/securedrop/source_templates/lookup.html
index ee9fc810d3..b9bf6fb933 100644
--- a/securedrop/source_templates/lookup.html
+++ b/securedrop/source_templates/lookup.html
@@ -54,12 +54,12 @@ <h2 class="headline">{{ gettext('Submit Messages') }}</h2>
 {% if allow_document_uploads %}
     <div class="attachment grid-item center">
       <img class="center" id="upload-icon" src="{{ url_for('static', filename='i/arrow-upload-large.png') }}" width="56" height="56">
-      <input type="file" name="fh" autocomplete="off">
+      {{ form.fh() }}
       <p class="center" id="max-file-size">{{ gettext('Maximum upload size: 500 MB') }}</p>
     </div>
 {% endif %}
     <div class="message grid-item{% if not allow_document_uploads %} wide{% endif %}">
-      <textarea name="msg" class="fill-parent" placeholder="{{ gettext('Write a message.') }}"></textarea>
+        {{ form.msg(class="fill-parent") }}
     </div>
   </div>
 
diff --git a/securedrop/tests/test_source.py b/securedrop/tests/test_source.py
index fa6c4b1b56..0313a6790f 100644
--- a/securedrop/tests/test_source.py
+++ b/securedrop/tests/test_source.py
@@ -352,11 +352,9 @@ def test_submit_empty_message(source_app):
 
 
 def test_submit_big_message(source_app):
-    '''
-    When the message is larger than 512KB it's written to disk instead of
-    just residing in memory. Make sure the different return type of
-    SecureTemporaryFile is handled as well as BytesIO.
-    '''
+    """
+    Test the message size limit.
+    """
     with source_app.test_client() as app:
         new_codename(app, session)
         _dummy_submission(app)
@@ -366,7 +364,7 @@ def test_submit_big_message(source_app):
             follow_redirects=True)
         assert resp.status_code == 200
         text = resp.data.decode('utf-8')
-        assert "Thanks! We received your message" in text
+        assert "Message text too long." in text
 
 
 def test_submit_file(source_app):