diff --git a/Lib/test/json_tests/test_dump.py b/Lib/test/json_tests/test_dump.py
index 083c11f..f1c1b04 100644
--- a/Lib/test/json_tests/test_dump.py
+++ b/Lib/test/json_tests/test_dump.py
@@ -1,6 +1,7 @@
 from io import StringIO
 from test.json_tests import PyTest, CTest
 
+from test.support import precisionbigmemtest, _1G
 
 class TestDump:
     def test_dump(self):
@@ -21,4 +22,20 @@
 
 
 class TestPyDump(TestDump, PyTest): pass
-class TestCDump(TestDump, CTest): pass
+
+class TestCDump(TestDump, CTest):
+
+    # The size requirement here is hopefully over-estimated (actual
+    # memory consumption depending on implementation details, and also
+    # system memory management, since this may allocate a lot of
+    # small objects).
+
+    @precisionbigmemtest(size=_1G, memuse=1)
+    def test_large_list(self, size):
+        N = int(30 * 1024 * 1024 * (size / _1G))
+        l = [1] * N
+        encoded = self.dumps(l)
+        self.assertEqual(len(encoded), N * 3)
+        self.assertEqual(encoded[:1], "[")
+        self.assertEqual(encoded[-2:], "1]")
+        self.assertEqual(encoded[1:-2], "1, " * (N - 1))
diff --git a/Misc/NEWS b/Misc/NEWS
index b96bb3d..36c9b8b 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -262,6 +262,9 @@
 Library
 -------
 
+- Issue #12778: Reduce memory consumption when JSON-encoding a large
+  container of many small objects.
+
 - Issue #12650: Fix a race condition where a subprocess.Popen could leak
   resources (FD/zombie) when killed at the wrong time.
 
diff --git a/Modules/_json.c b/Modules/_json.c
index a392c97..7c5e5e4 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -75,6 +75,121 @@
     {NULL}
 };
 
+/*
+ * A two-level accumulator of unicode objects that avoids both the overhead
+ * of keeping a huge number of small separate objects, and the quadratic
+ * behaviour of using a naive repeated concatenation scheme.
+ */
+
+typedef struct {
+    PyObject *large;  /* A list of previously accumulated large strings */
+    PyObject *small;  /* Pending small strings */
+} accumulator;
+
+static PyObject *
+join_list_unicode(PyObject *lst)
+{
+    /* return u''.join(lst) */
+    static PyObject *sep = NULL;
+    if (sep == NULL) {
+        sep = PyUnicode_FromStringAndSize("", 0);
+        if (sep == NULL)
+            return NULL;
+    }
+    return PyUnicode_Join(sep, lst);
+}
+
+static int
+init_accumulator(accumulator *acc)
+{
+    acc->large = PyList_New(0);
+    if (acc->large == NULL)
+        return -1;
+    acc->small = PyList_New(0);
+    if (acc->small == NULL) {
+        Py_CLEAR(acc->large);
+        return -1;
+    }
+    return 0;
+}
+
+static int
+flush_accumulator(accumulator *acc)
+{
+    Py_ssize_t nsmall = PyList_GET_SIZE(acc->small);
+    if (nsmall) {
+        int ret;
+        PyObject *joined = join_list_unicode(acc->small);
+        if (joined == NULL)
+            return -1;
+        if (PyList_SetSlice(acc->small, 0, nsmall, NULL)) {
+            Py_DECREF(joined);
+            return -1;
+        }
+        ret = PyList_Append(acc->large, joined);
+        Py_DECREF(joined);
+        return ret;
+    }
+    return 0;
+}
+
+static int
+accumulate_unicode(accumulator *acc, PyObject *obj)
+{
+    int ret;
+    Py_ssize_t nsmall;
+    assert(PyUnicode_Check(obj));
+
+    if (PyList_Append(acc->small, obj))
+        return -1;
+    nsmall = PyList_GET_SIZE(acc->small);
+    /* Each item in a list of unicode objects has an overhead (in 64-bit
+     * builds) of:
+     *   - 8 bytes for the list slot
+     *   - 56 bytes for the header of the unicode object
+     * that is, 64 bytes.  100000 such objects waste more than 6MB
+     * compared to a single concatenated string.
+     */
+    if (nsmall < 100000)
+        return 0;
+    PyObject *joined = join_list_unicode(acc->small);
+    if (joined == NULL)
+        return -1;
+    if (PyList_SetSlice(acc->small, 0, nsmall, NULL)) {
+        Py_DECREF(joined);
+        return -1;
+    }
+    ret = PyList_Append(acc->large, joined);
+    Py_DECREF(joined);
+    return ret;
+}
+
+static PyObject *
+finish_accumulator(accumulator *acc)
+{
+    int ret;
+    PyObject *res;
+
+    ret = flush_accumulator(acc);
+    Py_CLEAR(acc->small);
+    if (ret) {
+        Py_CLEAR(acc->large);
+        return NULL;
+    }
+    res = acc->large;
+    acc->large = NULL;
+    return res;
+}
+
+static void
+destroy_accumulator(accumulator *acc)
+{
+    Py_CLEAR(acc->small);
+    Py_CLEAR(acc->large);
+}
+
+/* Forward decls */
+
 static PyObject *
 ascii_escape_unicode(PyObject *pystr);
 static PyObject *
@@ -101,11 +216,11 @@
 static int
 encoder_clear(PyObject *self);
 static int
-encoder_listencode_list(PyEncoderObject *s, PyObject *rval, PyObject *seq, Py_ssize_t indent_level);
+encoder_listencode_list(PyEncoderObject *s, accumulator *acc, PyObject *seq, Py_ssize_t indent_level);
 static int
-encoder_listencode_obj(PyEncoderObject *s, PyObject *rval, PyObject *obj, Py_ssize_t indent_level);
+encoder_listencode_obj(PyEncoderObject *s, accumulator *acc, PyObject *obj, Py_ssize_t indent_level);
 static int
-encoder_listencode_dict(PyEncoderObject *s, PyObject *rval, PyObject *dct, Py_ssize_t indent_level);
+encoder_listencode_dict(PyEncoderObject *s, accumulator *acc, PyObject *dct, Py_ssize_t indent_level);
 static PyObject *
 _encoded_const(PyObject *obj);
 static void
@@ -267,19 +382,6 @@
 }
 
 static PyObject *
-join_list_unicode(PyObject *lst)
-{
-    /* return u''.join(lst) */
-    static PyObject *sep = NULL;
-    if (sep == NULL) {
-        sep = PyUnicode_FromStringAndSize("", 0);
-        if (sep == NULL)
-            return NULL;
-    }
-    return PyUnicode_Join(sep, lst);
-}
-
-static PyObject *
 _build_rval_index_tuple(PyObject *rval, Py_ssize_t idx) {
     /* return (rval, idx) tuple, stealing reference to rval */
     PyObject *tpl;
@@ -1226,22 +1328,22 @@
     /* Python callable interface to encode_listencode_obj */
     static char *kwlist[] = {"obj", "_current_indent_level", NULL};
     PyObject *obj;
-    PyObject *rval;
     Py_ssize_t indent_level;
     PyEncoderObject *s;
+    accumulator acc;
+
     assert(PyEncoder_Check(self));
     s = (PyEncoderObject *)self;
     if (!PyArg_ParseTupleAndKeywords(args, kwds, "OO&:_iterencode", kwlist,
         &obj, _convertPyInt_AsSsize_t, &indent_level))
         return NULL;
-    rval = PyList_New(0);
-    if (rval == NULL)
+    if (init_accumulator(&acc))
         return NULL;
-    if (encoder_listencode_obj(s, rval, obj, indent_level)) {
-        Py_DECREF(rval);
+    if (encoder_listencode_obj(s, &acc, obj, indent_level)) {
+        destroy_accumulator(&acc);
         return NULL;
     }
-    return rval;
+    return finish_accumulator(&acc);
 }
 
 static PyObject *
@@ -1313,18 +1415,19 @@
 }
 
 static int
-_steal_list_append(PyObject *lst, PyObject *stolen)
+_steal_accumulate(accumulator *acc, PyObject *stolen)
 {
     /* Append stolen and then decrement its reference count */
-    int rval = PyList_Append(lst, stolen);
+    int rval = accumulate_unicode(acc, stolen);
     Py_DECREF(stolen);
     return rval;
 }
 
 static int
-encoder_listencode_obj(PyEncoderObject *s, PyObject *rval, PyObject *obj, Py_ssize_t indent_level)
+encoder_listencode_obj(PyEncoderObject *s, accumulator *acc,
+                       PyObject *obj, Py_ssize_t indent_level)
 {
-    /* Encode Python object obj to a JSON term, rval is a PyList */
+    /* Encode Python object obj to a JSON term */
     PyObject *newobj;
     int rv;
 
@@ -1332,38 +1435,38 @@
         PyObject *cstr = _encoded_const(obj);
         if (cstr == NULL)
             return -1;
-        return _steal_list_append(rval, cstr);
+        return _steal_accumulate(acc, cstr);
     }
     else if (PyUnicode_Check(obj))
     {
         PyObject *encoded = encoder_encode_string(s, obj);
         if (encoded == NULL)
             return -1;
-        return _steal_list_append(rval, encoded);
+        return _steal_accumulate(acc, encoded);
     }
     else if (PyLong_Check(obj)) {
         PyObject *encoded = PyObject_Str(obj);
         if (encoded == NULL)
             return -1;
-        return _steal_list_append(rval, encoded);
+        return _steal_accumulate(acc, encoded);
     }
     else if (PyFloat_Check(obj)) {
         PyObject *encoded = encoder_encode_float(s, obj);
         if (encoded == NULL)
             return -1;
-        return _steal_list_append(rval, encoded);
+        return _steal_accumulate(acc, encoded);
     }
     else if (PyList_Check(obj) || PyTuple_Check(obj)) {
         if (Py_EnterRecursiveCall(" while encoding a JSON object"))
             return -1;
-        rv = encoder_listencode_list(s, rval, obj, indent_level);
+        rv = encoder_listencode_list(s, acc, obj, indent_level);
         Py_LeaveRecursiveCall();
         return rv;
     }
     else if (PyDict_Check(obj)) {
         if (Py_EnterRecursiveCall(" while encoding a JSON object"))
             return -1;
-        rv = encoder_listencode_dict(s, rval, obj, indent_level);
+        rv = encoder_listencode_dict(s, acc, obj, indent_level);
         Py_LeaveRecursiveCall();
         return rv;
     }
@@ -1394,7 +1497,7 @@
 
         if (Py_EnterRecursiveCall(" while encoding a JSON object"))
             return -1;
-        rv = encoder_listencode_obj(s, rval, newobj, indent_level);
+        rv = encoder_listencode_obj(s, acc, newobj, indent_level);
         Py_LeaveRecursiveCall();
 
         Py_DECREF(newobj);
@@ -1414,9 +1517,10 @@
 }
 
 static int
-encoder_listencode_dict(PyEncoderObject *s, PyObject *rval, PyObject *dct, Py_ssize_t indent_level)
+encoder_listencode_dict(PyEncoderObject *s, accumulator *acc,
+                        PyObject *dct, Py_ssize_t indent_level)
 {
-    /* Encode Python dict dct a JSON term, rval is a PyList */
+    /* Encode Python dict dct a JSON term */
     static PyObject *open_dict = NULL;
     static PyObject *close_dict = NULL;
     static PyObject *empty_dict = NULL;
@@ -1436,7 +1540,7 @@
             return -1;
     }
     if (Py_SIZE(dct) == 0)
-        return PyList_Append(rval, empty_dict);
+        return accumulate_unicode(acc, empty_dict);
 
     if (s->markers != Py_None) {
         int has_key;
@@ -1454,7 +1558,7 @@
         }
     }
 
-    if (PyList_Append(rval, open_dict))
+    if (accumulate_unicode(acc, open_dict))
         goto bail;
 
     if (s->indent != Py_None) {
@@ -1541,7 +1645,7 @@
         }
 
         if (idx) {
-            if (PyList_Append(rval, s->item_separator))
+            if (accumulate_unicode(acc, s->item_separator))
                 goto bail;
         }
 
@@ -1549,16 +1653,16 @@
         Py_CLEAR(kstr);
         if (encoded == NULL)
             goto bail;
-        if (PyList_Append(rval, encoded)) {
+        if (accumulate_unicode(acc, encoded)) {
             Py_DECREF(encoded);
             goto bail;
         }
         Py_DECREF(encoded);
-        if (PyList_Append(rval, s->key_separator))
+        if (accumulate_unicode(acc, s->key_separator))
             goto bail;
 
         value = PyTuple_GET_ITEM(item, 1);
-        if (encoder_listencode_obj(s, rval, value, indent_level))
+        if (encoder_listencode_obj(s, acc, value, indent_level))
             goto bail;
         idx += 1;
         Py_DECREF(item);
@@ -1578,7 +1682,7 @@
 
         yield '\n' + (' ' * (_indent * _current_indent_level))
     }*/
-    if (PyList_Append(rval, close_dict))
+    if (accumulate_unicode(acc, close_dict))
         goto bail;
     return 0;
 
@@ -1592,9 +1696,10 @@
 
 
 static int
-encoder_listencode_list(PyEncoderObject *s, PyObject *rval, PyObject *seq, Py_ssize_t indent_level)
+encoder_listencode_list(PyEncoderObject *s, accumulator *acc,
+                        PyObject *seq, Py_ssize_t indent_level)
 {
-    /* Encode Python list seq to a JSON term, rval is a PyList */
+    /* Encode Python list seq to a JSON term */
     static PyObject *open_array = NULL;
     static PyObject *close_array = NULL;
     static PyObject *empty_array = NULL;
@@ -1618,7 +1723,7 @@
     num_items = PySequence_Fast_GET_SIZE(s_fast);
     if (num_items == 0) {
         Py_DECREF(s_fast);
-        return PyList_Append(rval, empty_array);
+        return accumulate_unicode(acc, empty_array);
     }
 
     if (s->markers != Py_None) {
@@ -1638,7 +1743,7 @@
     }
 
     seq_items = PySequence_Fast_ITEMS(s_fast);
-    if (PyList_Append(rval, open_array))
+    if (accumulate_unicode(acc, open_array))
         goto bail;
     if (s->indent != Py_None) {
         /* TODO: DOES NOT RUN */
@@ -1652,10 +1757,10 @@
     for (i = 0; i < num_items; i++) {
         PyObject *obj = seq_items[i];
         if (i) {
-            if (PyList_Append(rval, s->item_separator))
+            if (accumulate_unicode(acc, s->item_separator))
                 goto bail;
         }
-        if (encoder_listencode_obj(s, rval, obj, indent_level))
+        if (encoder_listencode_obj(s, acc, obj, indent_level))
             goto bail;
     }
     if (ident != NULL) {
@@ -1670,7 +1775,7 @@
 
         yield '\n' + (' ' * (_indent * _current_indent_level))
     }*/
-    if (PyList_Append(rval, close_array))
+    if (accumulate_unicode(acc, close_array))
         goto bail;
     Py_DECREF(s_fast);
     return 0;