diff --git a/src/api/metadata/yaml/message.h b/src/api/metadata/yaml/message.h
index effb1f3e..8991082f 100644
--- a/src/api/metadata/yaml/message.h
+++ b/src/api/metadata/yaml/message.h
@@ -110,10 +110,38 @@ struct convert<loot::Message> {
         formatArgStore.push_back(sub);
       }
 
+      static const std::regex boostSyntax(
+          "%(\\d+)%", std::regex::ECMAScript | std::regex::icase);
+
       for (auto& mc : content) {
+        // Replace the old Boost.Format placeholder syntax for backward
+        // compatibility. To be removed after at least one major release of
+        // libloot to allow migration to the new syntax.
+        auto text = mc.GetText();
+        std::smatch match;
+        while (
+            std::regex_search(text.cbegin(), text.cend(), match, boostSyntax)) {
+          if (match.size() > 1) {
+            const auto n = std::stoul(match[1]);
+            if (n > 0) {
+              const auto newPlaceholder = "{" + std::to_string(n - 1) + "}";
+              text.replace(match[0].first, match[0].second, newPlaceholder);
+            } else {
+              throw RepresentationException(
+                  node.Mark(),
+                  "bad conversion: found zero-indexed placeholder using old "
+                  "syntax");
+            }
+          } else {
+            throw RepresentationException(node.Mark(),
+                                          "bad conversion: only partially "
+                                          "matched old placeholder syntax");
+          }
+        }
+
         try {
-          const auto text = fmt::vformat(mc.GetText(), formatArgStore);
-          mc = loot::MessageContent(text, mc.GetLanguage());
+          const auto formattedText = fmt::vformat(text, formatArgStore);
+          mc = loot::MessageContent(formattedText, mc.GetLanguage());
         } catch (const fmt::format_error& e) {
           throw RepresentationException(
               node.Mark(),
diff --git a/src/tests/api/internals/metadata/message_test.h b/src/tests/api/internals/metadata/message_test.h
index 0a6e4d8d..b531264e 100644
--- a/src/tests/api/internals/metadata/message_test.h
+++ b/src/tests/api/internals/metadata/message_test.h
@@ -669,6 +669,28 @@ TEST_P(MessageTest,
             message.GetContent());
 }
 
+TEST_P(MessageTest, decodingFromYamlShouldAcceptPercentagePlaceholderSyntax) {
+  YAML::Node node = YAML::Load(
+      "type: say\n"
+      "content: content %1% %2% %3% %4% %5% %6% %7% %8% %9% %10% %11%\n"
+      "subs:\n"
+      "  - a\n"
+      "  - b\n"
+      "  - c\n"
+      "  - d\n"
+      "  - e\n"
+      "  - f\n"
+      "  - g\n"
+      "  - h\n"
+      "  - i\n"
+      "  - j\n"
+      "  - k");
+  Message message = node.as<Message>();
+
+  ASSERT_EQ(1, message.GetContent().size());
+  EXPECT_EQ("content a b c d e f g h i j k", message.GetContent()[0].GetText());
+}
+
 TEST_P(MessageTest, decodingFromYamlShouldThrowIfAnInvalidConditionIsGiven) {
   YAML::Node node = YAML::Load(
       "type: say\n"