diff --git a/addons/event_sale/__manifest__.py b/addons/event_sale/__manifest__.py
index 2b08fa36c0a23..7e879a7f54be5 100644
--- a/addons/event_sale/__manifest__.py
+++ b/addons/event_sale/__manifest__.py
@@ -24,6 +24,7 @@
         'views/event_registration_views.xml',
         'views/event_views.xml',
         'views/sale_order_views.xml',
+        'views/templates.xml',
         'data/event_sale_data.xml',
         'data/mail_data.xml',
         'report/event_event_templates.xml',
diff --git a/addons/event_sale/models/sale_order.py b/addons/event_sale/models/sale_order.py
index 5e64c979fd69f..707b0eff78410 100644
--- a/addons/event_sale/models/sale_order.py
+++ b/addons/event_sale/models/sale_order.py
@@ -7,6 +7,7 @@ class SaleOrder(models.Model):
     _inherit = "sale.order"
 
     attendee_count = fields.Integer('Attendee Count', compute='_compute_attendee_count')
+    has_soldout_event = fields.Boolean(compute="_compute_has_soldout_event")
 
     def write(self, vals):
         """ Synchronize partner from SO to registrations. This is done notably
@@ -52,6 +53,21 @@ def _compute_attendee_count(self):
         for sale_order in self:
             sale_order.attendee_count = attendee_count_data.get(sale_order.id, 0)
 
+    def _compute_has_soldout_event(self):
+        """Meant to be used in the portal to prevent customers from paying for an event
+        that is sold out. Not only because we'd overbook the event, but also because it
+        will probably crash on the confirmation of the order when the registrations
+        are created."""
+        self.has_soldout_event = False
+        for order in self.filtered(lambda x: x.state in ["draft", "sent"]):
+            order.has_soldout_event = bool(order.order_line.event_id.filtered(
+                lambda x: x.seats_limited and x.seats_max and x.seats_available <= 0
+            ))
+
+    def has_to_be_paid(self, include_draft=False):
+        has_to_be_paid = super().has_to_be_paid(include_draft=include_draft)
+        return has_to_be_paid and not self.has_soldout_event
+
     def unlink(self):
         self.order_line._unlink_associated_registrations()
         return super(SaleOrder, self).unlink()
diff --git a/addons/event_sale/views/templates.xml b/addons/event_sale/views/templates.xml
new file mode 100644
index 0000000000000..7da78df7525e8
--- /dev/null
+++ b/addons/event_sale/views/templates.xml
@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="utf-8"?>
+<odoo>
+    <template id="sale_order_portal_template" inherit_id="sale.sale_order_portal_template" name="Event sale order portal">
+        <xpath expr="//div[@t-if='sale_order.is_expired']" position="before">
+            <div t-if="sale_order.has_soldout_event" class="alert alert-warning alert-dismissable d-print-none" role="alert">
+                <button type="button" class="close" data-dismiss="alert" aria-label="Close"></button>
+                <strong>The event is sold out!</strong> <a role="button" href="#discussion"><i class="fa fa-comment"/> Contact us for more information.</a>
+            </div>
+        </xpath>
+    </template>
+</odoo>