diff --git a/plugins/poll/config/locales/server.en.yml b/plugins/poll/config/locales/server.en.yml
index 1ef088e0e40..d3716baa435 100644
--- a/plugins/poll/config/locales/server.en.yml
+++ b/plugins/poll/config/locales/server.en.yml
@@ -41,6 +41,9 @@ en:
 
     requires_at_least_1_valid_option: "You must select at least 1 valid option."
 
+    default_cannot_be_made_public: "Poll with votes cannot be made public."
+    named_cannot_be_made_public: "Poll named <strong>%{name}</strong> has votes cannot be made public."
+
     cannot_change_polls_after_5_minutes: "You cannot add, remove or rename polls after the first 5 minutes."
     op_cannot_edit_options_after_5_minutes: "You cannot add or remove poll options after the first 5 minutes. Please contact a moderator if you need to edit a poll option."
     staff_cannot_add_or_remove_options_after_5_minutes: "You cannot add or remove poll options after the first 5 minutes. You should close this topic and create a new one instead."
diff --git a/plugins/poll/lib/polls_updater.rb b/plugins/poll/lib/polls_updater.rb
index 2f05f91c706..df3b22432d9 100644
--- a/plugins/poll/lib/polls_updater.rb
+++ b/plugins/poll/lib/polls_updater.rb
@@ -41,6 +41,7 @@ module DiscoursePoll
         # try to merge votes
         polls.each_key do |poll_name|
           next unless previous_polls.has_key?(poll_name)
+          return if has_votes && private_to_public_poll?(post, previous_polls, polls, poll_name)
 
           # when the # of options has changed, reset all the votes
           if polls[poll_name]["options"].size != previous_polls[poll_name]["options"].size
@@ -96,5 +97,26 @@ module DiscoursePoll
     def self.total_votes(polls)
       polls.map { |key, value| value["voters"].to_i }.sum
     end
+
+    private
+
+    def self.private_to_public_poll?(post, previous_polls, current_polls, poll_name)
+      previous_poll = previous_polls[poll_name]
+      current_poll = current_polls[poll_name]
+
+      if previous_polls["public"].nil? && current_poll["public"] == "true"
+        error =
+          if poll_name == DiscoursePoll::DEFAULT_POLL_NAME
+            I18n.t("poll.default_cannot_be_made_public")
+          else
+            I18n.t("poll.named_cannot_be_made_public", name: poll_name)
+          end
+
+        post.errors.add(:base, error)
+        return true
+      end
+
+      false
+    end
   end
 end
diff --git a/plugins/poll/spec/lib/polls_updater_spec.rb b/plugins/poll/spec/lib/polls_updater_spec.rb
index d2b89c0911a..2dba772e843 100644
--- a/plugins/poll/spec/lib/polls_updater_spec.rb
+++ b/plugins/poll/spec/lib/polls_updater_spec.rb
@@ -101,7 +101,7 @@ describe DiscoursePoll::PollsUpdater do
         Fabricate(:post, raw: raw)
       end
 
-      let(:private_poll) do
+      let(:private_poll_post) do
         raw = <<-RAW.strip_heredoc
         [poll]
         - A
@@ -109,7 +109,11 @@ describe DiscoursePoll::PollsUpdater do
         [/poll]
         RAW
 
-        DiscoursePoll::PollsValidator.new(Fabricate(:post, raw: raw)).validate_polls
+        Fabricate(:post, raw: raw)
+      end
+
+      let(:private_poll) do
+        DiscoursePoll::PollsValidator.new(private_poll_post).validate_polls
       end
 
       let(:public_poll) do
@@ -130,6 +134,21 @@ describe DiscoursePoll::PollsUpdater do
         post.reload
       end
 
+      it "should not allow a private poll with votes to be made public" do
+        DiscoursePoll::Poll.vote(private_poll_post.id, "poll", ["5c24fc1df56d764b550ceae1b9319125"], user.id)
+        private_poll_post.reload
+
+        messages = MessageBus.track_publish do
+          described_class.update(private_poll_post, public_poll)
+        end
+
+        expect(messages).to eq([])
+
+        expect(private_poll_post.errors[:base]).to include(
+          I18n.t("poll.default_cannot_be_made_public")
+        )
+      end
+
       it "should retain voter_ids when options have been edited" do
         described_class.update(post, public_poll)