diff --git a/src/Storages/MergeTree/MergeTreeDataMergerMutator.cpp b/src/Storages/MergeTree/MergeTreeDataMergerMutator.cpp
index 8b3c7bdf3fb..cb67c6caf7f 100644
--- a/src/Storages/MergeTree/MergeTreeDataMergerMutator.cpp
+++ b/src/Storages/MergeTree/MergeTreeDataMergerMutator.cpp
@@ -437,6 +437,7 @@ MergeTreeDataMergerMutator::MergeSelectingInfo MergeTreeDataMergerMutator::getPo
 
         auto & partition_info = partitions_info[partition_id];
         partition_info.min_age = std::min(partition_info.min_age, part_info.age);
+        ++partition_info.num_parts;
 
         ++res.parts_selected_precondition;
 
@@ -606,11 +607,21 @@ String MergeTreeDataMergerMutator::getBestPartitionToOptimizeEntire(
     auto best_partition_it = std::max_element(
         partitions_info.begin(),
         partitions_info.end(),
-        [](const auto & e1, const auto & e2) { return e1.second.min_age < e2.second.min_age; });
+        [](const auto & e1, const auto & e2)
+        {
+            // If one partition has only a single part, always select the other partition.
+            if (e1.second.num_parts == 1)
+                return true;
+            if (e2.second.num_parts == 1)
+                return false;
+            // If both partitions have more than one part, select the older partition.
+            return e1.second.min_age < e2.second.min_age;
+        });
 
     assert(best_partition_it != partitions_info.end());
 
-    if (static_cast<size_t>(best_partition_it->second.min_age) < (*data_settings)[MergeTreeSetting::min_age_to_force_merge_seconds])
+    if ((static_cast<size_t>(best_partition_it->second.min_age) < (*data_settings)[MergeTreeSetting::min_age_to_force_merge_seconds])
+        || static_cast<size_t>(best_partition_it->second.num_parts) == 1)
         return {};
 
     return best_partition_it->first;
diff --git a/src/Storages/MergeTree/MergeTreeDataMergerMutator.h b/src/Storages/MergeTree/MergeTreeDataMergerMutator.h
index 71fcb93f369..5920a832c19 100644
--- a/src/Storages/MergeTree/MergeTreeDataMergerMutator.h
+++ b/src/Storages/MergeTree/MergeTreeDataMergerMutator.h
@@ -65,6 +65,7 @@ public:
     struct PartitionInfo
     {
         time_t min_age{std::numeric_limits<time_t>::max()};
+        size_t num_parts = 0;
     };
     using PartitionsInfo = std::unordered_map<std::string, PartitionInfo>;
 
diff --git a/tests/queries/0_stateless/02473_optimize_old_parts.reference b/tests/queries/0_stateless/02473_optimize_old_parts.reference
index 7d08cd5bbef..19526022e88 100644
--- a/tests/queries/0_stateless/02473_optimize_old_parts.reference
+++ b/tests/queries/0_stateless/02473_optimize_old_parts.reference
@@ -3,5 +3,6 @@ Without merge
 With merge any part range
 1
 With merge partition only
+2
 1
 1
diff --git a/tests/queries/0_stateless/02473_optimize_old_parts.sh b/tests/queries/0_stateless/02473_optimize_old_parts.sh
index 3a4e6145f12..6c6d484d8d9 100755
--- a/tests/queries/0_stateless/02473_optimize_old_parts.sh
+++ b/tests/queries/0_stateless/02473_optimize_old_parts.sh
@@ -52,13 +52,13 @@ DROP TABLE test_with_merge;
 
 SELECT 'With merge partition only';
 
-CREATE TABLE test_with_merge (i Int64) ENGINE = MergeTree ORDER BY i
+CREATE TABLE test_with_merge (i Int64) ENGINE = MergeTree ORDER BY i PARTITION BY i
 SETTINGS min_age_to_force_merge_seconds=1, merge_selecting_sleep_ms=1000, min_age_to_force_merge_on_partition_only=true;
 INSERT INTO test_with_merge SELECT 1;
 INSERT INTO test_with_merge SELECT 2;
-INSERT INTO test_with_merge SELECT 3;"
+INSERT INTO test_with_merge SELECT 2 SETTINGS insert_deduplicate = 0;"
 
-wait_for_number_of_parts 'test_with_merge' 1 100
+wait_for_number_of_parts 'test_with_merge' 2 100
 
 $CLICKHOUSE_CLIENT -mq "
 SELECT sleepEachRow(1) FROM numbers(9) SETTINGS function_sleep_max_microseconds_per_block = 10000000 FORMAT Null; -- Sleep for 9 seconds and verify that we keep the old part because it's the only one
diff --git a/tests/queries/0_stateless/02676_optimize_old_parts_replicated.reference b/tests/queries/0_stateless/02676_optimize_old_parts_replicated.reference
index 0f3c482f188..46f409a3a73 100644
--- a/tests/queries/0_stateless/02676_optimize_old_parts_replicated.reference
+++ b/tests/queries/0_stateless/02676_optimize_old_parts_replicated.reference
@@ -3,5 +3,6 @@ Without merge
 With merge replicated any part range
 1
 With merge replicated partition only
+2
 1
 1
diff --git a/tests/queries/0_stateless/02676_optimize_old_parts_replicated.sh b/tests/queries/0_stateless/02676_optimize_old_parts_replicated.sh
index 84be5959344..39a6e09bc96 100755
--- a/tests/queries/0_stateless/02676_optimize_old_parts_replicated.sh
+++ b/tests/queries/0_stateless/02676_optimize_old_parts_replicated.sh
@@ -53,12 +53,14 @@ DROP TABLE test_replicated;
 SELECT 'With merge replicated partition only';
 
 CREATE TABLE test_replicated (i Int64) ENGINE = ReplicatedMergeTree('/clickhouse/tables/{database}/test02676_partition_only', 'node')  ORDER BY i
+PARTITION BY i
 SETTINGS min_age_to_force_merge_seconds=1, merge_selecting_sleep_ms=1000, min_age_to_force_merge_on_partition_only=true;
 INSERT INTO test_replicated SELECT 1;
 INSERT INTO test_replicated SELECT 2;
-INSERT INTO test_replicated SELECT 3;"
+SELECT sleep(3) FORMAT Null; -- Sleep so the first partition is older
+INSERT INTO test_replicated SELECT 2 SETTINGS insert_deduplicate = 0;"
 
-wait_for_number_of_parts 'test_replicated' 1 100
+wait_for_number_of_parts 'test_replicated' 2 100
 
 $CLICKHOUSE_CLIENT -mq "
 SELECT sleepEachRow(1) FROM numbers(9) SETTINGS function_sleep_max_microseconds_per_block = 10000000 FORMAT Null; -- Sleep for 9 seconds and verify that we keep the old part because it's the only one