diff --git a/Modules/_json.c b/Modules/_json.c
index 89b0a41dd10acb..82e5dc7e7fd909 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -51,7 +51,7 @@ typedef struct _PyEncoderObject {
     char sort_keys;
     char skipkeys;
     int allow_nan;
-    PyCFunction fast_encode;
+    PyObject * (*fast_encode)(PyObject *);
 } PyEncoderObject;
 
 #define PyEncoderObject_CAST(op)    ((PyEncoderObject *)(op))
@@ -102,8 +102,8 @@ static PyObject *
 _encoded_const(PyObject *obj);
 static void
 raise_errmsg(const char *msg, PyObject *s, Py_ssize_t end);
-static PyObject *
-encoder_encode_string(PyEncoderObject *s, PyObject *obj);
+static int
+encoder_write_string(PyEncoderObject *s, PyUnicodeWriter *writer, PyObject *obj);
 static PyObject *
 encoder_encode_float(PyEncoderObject *s, PyObject *obj);
 
@@ -209,6 +209,72 @@ ascii_escape_unicode(PyObject *pystr)
     return rval;
 }
 
+static PyObject *
+ascii_escape_unicode_ex(PyObject *pystr)
+{
+    /* Take a PyUnicode pystr and return a new ASCII-only escaped PyUnicode */
+    Py_ssize_t i;
+    Py_ssize_t input_chars;
+    Py_ssize_t output_size;
+    Py_ssize_t chars;
+    PyObject *rval;
+    const void *input;
+    Py_UCS1 *output;
+    int kind;
+
+    input_chars = PyUnicode_GET_LENGTH(pystr);
+    input = PyUnicode_DATA(pystr);
+    kind = PyUnicode_KIND(pystr);
+
+    /* Compute the output size */
+    for (i = 0, output_size = 0; i < input_chars; i++) {
+        Py_UCS4 c = PyUnicode_READ(kind, input, i);
+        Py_ssize_t d;
+        if (S_CHAR(c)) {
+            d = 1;
+        }
+        else {
+            switch(c) {
+            case '\\': case '"': case '\b': case '\f':
+            case '\n': case '\r': case '\t':
+                d = 2; break;
+            default:
+                d = c >= 0x10000 ? 12 : 6;
+            }
+        }
+        if (output_size > PY_SSIZE_T_MAX - d) {
+            PyErr_SetString(PyExc_OverflowError, "string is too long to escape");
+            return NULL;
+        }
+        output_size += d;
+    }
+
+    if (output_size == input_chars) {
+        /* No need to escape anything */
+        return Py_NewRef(pystr);
+    }
+
+    rval = PyUnicode_New(output_size, 127);
+    if (rval == NULL) {
+        return NULL;
+    }
+    output = PyUnicode_1BYTE_DATA(rval);
+    chars = 0;
+    for (i = 0; i < input_chars; i++) {
+        Py_UCS4 c = PyUnicode_READ(kind, input, i);
+        if (S_CHAR(c)) {
+            output[chars++] = c;
+        }
+        else {
+            chars = ascii_escape_unichar(c, output, chars);
+        }
+    }
+#ifdef Py_DEBUG
+    assert(_PyUnicode_CheckConsistency(rval, 1));
+#endif
+    return rval;
+}
+
 static PyObject *
 escape_unicode(PyObject *pystr)
 {
@@ -303,6 +369,103 @@ escape_unicode(PyObject *pystr)
     return rval;
 }
 
+static PyObject *
+escape_unicode_ex(PyObject *pystr)
+{
+    /* Take a PyUnicode pystr and return a new escaped PyUnicode */
+    Py_ssize_t i;
+    Py_ssize_t input_chars;
+    Py_ssize_t output_size;
+    Py_ssize_t chars;
+    PyObject *rval;
+    const void *input;
+    int kind;
+    Py_UCS4 maxchar;
+
+    maxchar = PyUnicode_MAX_CHAR_VALUE(pystr);
+    input_chars = PyUnicode_GET_LENGTH(pystr);
+    input = PyUnicode_DATA(pystr);
+    kind = PyUnicode_KIND(pystr);
+
+    /* Compute the output size */
+    for (i = 0, output_size = 0; i < input_chars; i++) {
+        Py_UCS4 c = PyUnicode_READ(kind, input, i);
+        Py_ssize_t d;
+        switch (c) {
+        case '\\': case '"': case '\b': case '\f':
+        case '\n': case '\r': case '\t':
+            d = 2;
+            break;
+        default:
+            if (c <= 0x1f)
+                d = 6;
+            else
+                d = 1;
+        }
+        if (output_size > PY_SSIZE_T_MAX - d) {
+            PyErr_SetString(PyExc_OverflowError, "string is too long to escape");
+            return NULL;
+        }
+        output_size += d;
+    }
+
+    if (output_size == input_chars) {
+        /* No need to escape anything */
+        return Py_NewRef(pystr);
+    }
+
+    rval = PyUnicode_New(output_size, maxchar);
+    if (rval == NULL)
+        return NULL;
+
+    kind = PyUnicode_KIND(rval);
+
+#define ENCODE_OUTPUT do { \
+        chars = 0; \
+        for (i = 0; i < input_chars; i++) { \
+            Py_UCS4 c = PyUnicode_READ(kind, input, i); \
+            switch (c) { \
+            case '\\': output[chars++] = '\\'; output[chars++] = c; break; \
+            case '"':  output[chars++] = '\\'; output[chars++] = c; break; \
+            case '\b': output[chars++] = '\\'; output[chars++] = 'b'; break; \
+            case '\f': output[chars++] = '\\'; output[chars++] = 'f'; break; \
+            case '\n': output[chars++] = '\\'; output[chars++] = 'n'; break; \
+            case '\r': output[chars++] = '\\'; output[chars++] = 'r'; break; \
+            case '\t': output[chars++] = '\\'; output[chars++] = 't'; break; \
+            default: \
+                if (c <= 0x1f) { \
+                    output[chars++] = '\\'; \
+                    output[chars++] = 'u'; \
+                    output[chars++] = '0'; \
+                    output[chars++] = '0'; \
+                    output[chars++] = Py_hexdigits[(c >> 4) & 0xf]; \
+                    output[chars++] = Py_hexdigits[(c     ) & 0xf]; \
+                } else { \
+                    output[chars++] = c; \
+                } \
+            } \
+        } \
+    } while (0)
+
+    if (kind == PyUnicode_1BYTE_KIND) {
+        Py_UCS1 *output = PyUnicode_1BYTE_DATA(rval);
+        ENCODE_OUTPUT;
+    } else if (kind == PyUnicode_2BYTE_KIND) {
+        Py_UCS2 *output = PyUnicode_2BYTE_DATA(rval);
+        ENCODE_OUTPUT;
+    } else {
+        Py_UCS4 *output = PyUnicode_4BYTE_DATA(rval);
+        assert(kind == PyUnicode_4BYTE_KIND);
+        ENCODE_OUTPUT;
+    }
+#undef ENCODE_OUTPUT
+
+#ifdef Py_DEBUG
+    assert(_PyUnicode_CheckConsistency(rval, 1));
+#endif
+    return rval;
+}
+
 static void
 raise_errmsg(const char *msg, PyObject *s, Py_ssize_t end)
 {
@@ -1255,8 +1418,11 @@ encoder_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 
     if (PyCFunction_Check(s->encoder)) {
         PyCFunction f = PyCFunction_GetFunction(s->encoder);
-        if (f == py_encode_basestring_ascii || f == py_encode_basestring) {
-            s->fast_encode = f;
+        if (f == py_encode_basestring_ascii) {
+            s->fast_encode = ascii_escape_unicode_ex;
+        }
+        else if (f == py_encode_basestring) {
+            s->fast_encode = escape_unicode_ex;
         }
     }
 
@@ -1437,33 +1603,46 @@ encoder_encode_float(PyEncoderObject *s, PyObject *obj)
     return PyFloat_Type.tp_repr(obj);
 }
 
-static PyObject *
-encoder_encode_string(PyEncoderObject *s, PyObject *obj)
+static int
+_steal_accumulate(PyUnicodeWriter *writer, PyObject *stolen)
+{
+    /* Append stolen and then decrement its reference count */
+    int rval = PyUnicodeWriter_WriteStr(writer, stolen);
+    Py_DECREF(stolen);
+    return rval;
+}
+
+static int
+encoder_write_string(PyEncoderObject *s, PyUnicodeWriter *writer, PyObject *obj)
 {
     /* Return the JSON representation of a string */
     PyObject *encoded;
 
     if (s->fast_encode) {
-        return s->fast_encode(NULL, obj);
+        if (PyUnicodeWriter_WriteChar(writer, '"') < 0) {
+            return -1;
+        }
+        encoded = s->fast_encode(obj);
+        if (encoded == NULL) {
+            return -1;
+        }
+        if (_steal_accumulate(writer, encoded) < 0) {
+            return -1;
+        }
+        return PyUnicodeWriter_WriteChar(writer, '"');
     }
     encoded = PyObject_CallOneArg(s->encoder, obj);
-    if (encoded != NULL && !PyUnicode_Check(encoded)) {
+    if (encoded == NULL) {
+        return -1;
+    }
+    if (!PyUnicode_Check(encoded)) {
         PyErr_Format(PyExc_TypeError,
                      "encoder() must return a string, not %.80s",
                      Py_TYPE(encoded)->tp_name);
         Py_DECREF(encoded);
-        return NULL;
+        return -1;
     }
-    return encoded;
-}
-
-static int
-_steal_accumulate(PyUnicodeWriter *writer, PyObject *stolen)
-{
-    /* Append stolen and then decrement its reference count */
-    int rval = PyUnicodeWriter_WriteStr(writer, stolen);
-    Py_DECREF(stolen);
-    return rval;
+    return _steal_accumulate(writer, encoded);
 }
 
 static int
@@ -1485,10 +1664,7 @@ encoder_listencode_obj(PyEncoderObject *s, PyUnicodeWriter *writer,
       return PyUnicodeWriter_WriteUTF8(writer, "false", 5);
     }
     else if (PyUnicode_Check(obj)) {
-        PyObject *encoded = encoder_encode_string(s, obj);
-        if (encoded == NULL)
-            return -1;
-        return _steal_accumulate(writer, encoded);
+        return encoder_write_string(s, writer, obj);
     }
     else if (PyLong_Check(obj)) {
         if (PyLong_CheckExact(obj)) {
@@ -1577,7 +1753,7 @@ encoder_encode_key_value(PyEncoderObject *s, PyUnicodeWriter *writer, bool *firs
                          PyObject *item_separator)
 {
     PyObject *keystr = NULL;
-    PyObject *encoded;
+    int rv;
 
     if (PyUnicode_Check(key)) {
         keystr = Py_NewRef(key);
@@ -1617,14 +1793,11 @@ encoder_encode_key_value(PyEncoderObject *s, PyUnicodeWriter *writer, bool *firs
         }
     }
 
-    encoded = encoder_encode_string(s, keystr);
+    rv = encoder_write_string(s, writer, keystr);
     Py_DECREF(keystr);
-    if (encoded == NULL) {
-        return -1;
-    }
 
-    if (_steal_accumulate(writer, encoded) < 0) {
-        return -1;
+    if (rv < 0) {
+        return rv;
     }
     if (PyUnicodeWriter_WriteStr(writer, s->key_separator) < 0) {
         return -1;