diff --git a/src/Storages/MergeTree/MergeTreeData.cpp b/src/Storages/MergeTree/MergeTreeData.cpp
index 9350b24c96a..fbe79a32b8e 100644
--- a/src/Storages/MergeTree/MergeTreeData.cpp
+++ b/src/Storages/MergeTree/MergeTreeData.cpp
@@ -5361,20 +5361,50 @@ MergeTreeData::PartsBackupEntries MergeTreeData::backupParts(
             &temp_dirs,
             false, false);
 
-        auto projection_parts = part->getProjectionParts();
-        for (const auto & [projection_name, projection_part] : projection_parts)
+        auto backup_projection = [&](IDataPartStorage & storage, IMergeTreeDataPart & projection_part)
         {
-            projection_part->getDataPartStorage().backup(
-                projection_part->checksums,
-                projection_part->getFileNamesWithoutChecksums(),
+            storage.backup(
+                projection_part.checksums,
+                projection_part.getFileNamesWithoutChecksums(),
                 fs::path{data_path_in_backup} / part->name,
                 backup_settings,
                 read_settings,
                 make_temporary_hard_links,
                 backup_entries_from_part,
                 &temp_dirs,
-                projection_part->is_broken,
+                projection_part.is_broken,
                 backup_settings.allow_backup_broken_projections);
+        };
+
+        auto projection_parts = part->getProjectionParts();
+        std::string proj_suffix = ".proj";
+        std::unordered_set<String> defined_projections;
+
+        for (const auto & [projection_name, projection_part] : projection_parts)
+        {
+            defined_projections.emplace(projection_name);
+            backup_projection(projection_part->getDataPartStorage(), *projection_part);
+        }
+
+        /// It is possible that the part has a written but not loaded projection,
+        /// e.g. it is written to parent part's checksums.txt and exists on disk,
+        /// but does not exist in table's projections definition.
+        /// Such a part can appear server was restarted after DROP PROJECTION but before old part was removed.
+        /// In this case, the old part will load only projections from metadata.
+        /// See 031145_non_loaded_projection_backup.sh.
+        for (const auto & [name, _] : part->checksums.files)
+        {
+            auto projection_name = fs::path(name).stem().string();
+            if (endsWith(name, proj_suffix) && !defined_projections.contains(projection_name))
+            {
+                auto projection_storage = part->getDataPartStorage().getProjection(projection_name + proj_suffix);
+                if (projection_storage->exists("checksums.txt"))
+                {
+                    auto projection_part = const_cast<IMergeTreeDataPart &>(*part).getProjectionPartBuilder(
+                        projection_name, /* is_temp_projection */false).withPartFormatFromDisk().build();
+                    backup_projection(projection_part->getDataPartStorage(), *projection_part);
+                }
+            }
         }
 
         if (hold_storage_and_part_ptrs)
diff --git a/tests/queries/0_stateless/03145_non_loaded_projection_backup.reference b/tests/queries/0_stateless/03145_non_loaded_projection_backup.reference
new file mode 100644
index 00000000000..a11ee210e62
--- /dev/null
+++ b/tests/queries/0_stateless/03145_non_loaded_projection_backup.reference
@@ -0,0 +1,7 @@
+7
+Found unexpected projection directories: pp.proj
+BACKUP_CREATED
+RESTORED
+7
+Found unexpected projection directories: pp.proj
+0
diff --git a/tests/queries/0_stateless/03145_non_loaded_projection_backup.sh b/tests/queries/0_stateless/03145_non_loaded_projection_backup.sh
new file mode 100755
index 00000000000..721ed784fc2
--- /dev/null
+++ b/tests/queries/0_stateless/03145_non_loaded_projection_backup.sh
@@ -0,0 +1,49 @@
+#!/usr/bin/env bash
+
+CURDIR=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+# shellcheck source=../shell_config.sh
+. "$CURDIR"/../shell_config.sh
+
+$CLICKHOUSE_CLIENT -nm -q "
+drop table if exists tp_1;
+create table tp_1 (x Int32, y Int32, projection p (select x, y order by x)) engine = MergeTree order by y partition by intDiv(y, 100);
+insert into tp_1 select number, number from numbers(3);
+
+set mutations_sync = 2;
+
+alter table tp_1 add projection pp (select x, count() group by x);
+insert into tp_1 select number, number from numbers(4);
+select count() from tp_1;
+
+-- Here we have a part with written projection pp
+alter table tp_1 detach partition '0';
+-- Move part to detached
+alter table tp_1 clear projection pp;
+-- Remove projection from table metadata
+alter table tp_1 drop projection pp;
+-- Now, we don't load projection pp for attached part, but it is written on disk
+alter table tp_1 attach partition '0';
+"
+
+$CLICKHOUSE_CLIENT -nm -q "
+set send_logs_level='fatal';
+check table tp_1 settings check_query_single_value_result = 0;" | grep -o "Found unexpected projection directories: pp.proj"
+
+backup_id="$CLICKHOUSE_TEST_UNIQUE_NAME"
+$CLICKHOUSE_CLIENT -q "
+backup table tp_1 to Disk('backups', '$backup_id');
+" | grep -o "BACKUP_CREATED"
+
+$CLICKHOUSE_CLIENT -nm -q "
+drop table tp_1;
+restore table tp_1 from Disk('backups', '$backup_id');
+" | grep -o "RESTORED"
+
+$CLICKHOUSE_CLIENT -q "select count() from tp_1;"
+$CLICKHOUSE_CLIENT -nm -q "
+set send_logs_level='fatal';
+check table tp_1 settings check_query_single_value_result = 0;" | grep -o "Found unexpected projection directories: pp.proj"
+$CLICKHOUSE_CLIENT -nm -q "
+set send_logs_level='fatal';
+check table tp_1"
+$CLICKHOUSE_CLIENT -q "drop table tp_1 sync"