diff --git a/src/spikeinterface/postprocessing/correlograms.py b/src/spikeinterface/postprocessing/correlograms.py
index ba12a5c462..ac2e43469a 100644
--- a/src/spikeinterface/postprocessing/correlograms.py
+++ b/src/spikeinterface/postprocessing/correlograms.py
@@ -3,6 +3,7 @@
 import warnings
 import numpy as np
 from spikeinterface.core.sortinganalyzer import register_result_extension, AnalyzerExtension, SortingAnalyzer
+from copy import deepcopy
 
 from spikeinterface.core.waveforms_extractor_backwards_compatibility import MockWaveformExtractor
 
@@ -93,9 +94,46 @@ def _select_extension_data(self, unit_ids):
     def _merge_extension_data(
         self, merge_unit_groups, new_unit_ids, new_sorting_analyzer, censor_ms=None, verbose=False, **job_kwargs
     ):
-        # recomputing correlogram is fast enough and much easier in this case
-        new_ccgs, new_bins = _compute_correlograms_on_sorting(new_sorting_analyzer.sorting, **self.params)
-        new_data = dict(ccgs=new_ccgs, bins=new_bins)
+
+        method = "keep_first"
+        cut_from = 1
+
+        correlograms, new_bins = deepcopy(self.get_data())
+
+        for new_unit_id, merge_unit_group in zip(new_unit_ids, merge_unit_groups):
+
+            merge_unit_group_indices = self.sorting_analyzer.sorting.ids_to_indices(merge_unit_group)
+
+            new_col = np.sum(correlograms[merge_unit_group_indices, :, :], axis=0)
+            correlograms[merge_unit_group_indices[0], :, :] = new_col
+            correlograms[merge_unit_group_indices[1:], :, :] = 0
+
+            new_row = np.sum(correlograms[:, merge_unit_group_indices, :], axis=1)
+            correlograms[:, merge_unit_group_indices[0], :] = new_row
+            correlograms[:, merge_unit_group_indices[1:], :] = 0
+
+            if new_unit_id not in merge_unit_group:
+                method = "append"
+                cut_from = 0
+
+        if method == "append":
+            old_num_units = np.shape(correlograms)[0]
+            correlograms = np.pad(correlograms, ((0, len(new_unit_ids)), (0, len(new_unit_ids)), (0, 0)))
+            for a, (new_unit_id, merge_unit_group) in enumerate(zip(new_unit_ids, merge_unit_groups)):
+
+                old_loc = self.sorting_analyzer.sorting.ids_to_indices([merge_unit_group[0]])[0]
+                new_loc = old_num_units + a
+
+                correlograms[:, [old_loc, new_loc], :] = correlograms[:, [new_loc, old_loc], :]
+                correlograms[[old_loc, new_loc], :, :] = correlograms[[new_loc, old_loc], :, :]
+
+        units_to_delete = np.concatenate([merge_unit_group[cut_from:] for merge_unit_group in merge_unit_groups])
+        indices_to_delete = self.sorting_analyzer.sorting.ids_to_indices(units_to_delete)
+
+        correlograms = np.delete(correlograms, indices_to_delete, axis=0)
+        correlograms = np.delete(correlograms, indices_to_delete, axis=1)
+
+        new_data = dict(ccgs=correlograms, bins=new_bins)
         return new_data
 
     def _run(self, verbose=False):
diff --git a/src/spikeinterface/postprocessing/tests/test_extension_merges.py b/src/spikeinterface/postprocessing/tests/test_extension_merges.py
new file mode 100644
index 0000000000..b42d5d84b4
--- /dev/null
+++ b/src/spikeinterface/postprocessing/tests/test_extension_merges.py
@@ -0,0 +1,36 @@
+import numpy as np
+
+from spikeinterface.core import generate_ground_truth_recording, create_sorting_analyzer
+
+
+def test_correlograms_merge():
+    """
+    When merging in `soft` mode, correlograms sum and we take advantage of this to make
+    a fast computation. This test checks that we get the same result using this fast
+    sum as recomputing the correlograms from scratch.
+    """
+
+    rec, sort = generate_ground_truth_recording()
+
+    sorting_analyzer = create_sorting_analyzer(recording=rec, sorting=sort)
+    sorting_analyzer.compute("correlograms")
+
+    trial_merges = [
+        [["1", "2"]],
+        [["2", "4", "6", "8"]],
+        [["1", "4", "7"], ["2", "8"]],
+        [["4", "1", "8"], ["2", "7", "0"], ["3", "9"], ["5", "6"]],
+    ]
+
+    for new_id_strategy in ["append", "take_first"]:
+        for merge_unit_groups in trial_merges:
+
+            # first, compute the correlograms of the merged units using the merge method
+            merged_sorting_analyzer = sorting_analyzer.merge_units(
+                merge_unit_groups=merge_unit_groups, new_id_strategy=new_id_strategy
+            )
+            computed_correlograms = merged_sorting_analyzer.get_extension("correlograms").get_data()
+
+            # Then re-compute, and compare
+            recomputed_correlograms = merged_sorting_analyzer.compute("correlograms").get_data()
+            assert np.all(computed_correlograms[0] == recomputed_correlograms[0])