diff --git a/docs/en/sql-reference/functions/type-conversion-functions.md b/docs/en/sql-reference/functions/type-conversion-functions.md
index 7cc2c022143..dc90697bd20 100644
--- a/docs/en/sql-reference/functions/type-conversion-functions.md
+++ b/docs/en/sql-reference/functions/type-conversion-functions.md
@@ -3467,13 +3467,13 @@ This function is the opposite operation of function [formatDateTime](../function
 **Syntax**
 
 ``` sql
-parseDateTime(str, format[, timezone])
+parseDateTime(str[, format[, timezone]])
 ```
 
 **Arguments**
 
-- `str` — the String to be parsed
-- `format` — the format string
+- `str` — The String to be parsed
+- `format` — The format string. Optional. `%Y-%m-%d %H:%i:%s` if not specified.
 - `timezone` — [Timezone](/docs/en/operations/server-configuration-parameters/settings.md/#server_configuration_parameters-timezone). Optional.
 
 **Returned value(s)**
@@ -3516,13 +3516,13 @@ This function is the opposite operation of function [formatDateTimeInJodaSyntax]
 **Syntax**
 
 ``` sql
-parseDateTimeInJodaSyntax(str, format[, timezone])
+parseDateTimeInJodaSyntax(str[, format[, timezone]])
 ```
 
 **Arguments**
 
-- `str` — the String to be parsed
-- `format` — the format string
+- `str` — The String to be parsed
+- `format` — The format string. Optional. `yyyy-MM-dd HH:mm:ss` if not specified.
 - `timezone` — [Timezone](/docs/en/operations/server-configuration-parameters/settings.md/#server_configuration_parameters-timezone). Optional.
 
 **Returned value(s)**
diff --git a/src/Functions/parseDateTime.cpp b/src/Functions/parseDateTime.cpp
index bdca0151bba..7ca10677be7 100644
--- a/src/Functions/parseDateTime.cpp
+++ b/src/Functions/parseDateTime.cpp
@@ -582,11 +582,11 @@ namespace
         DataTypePtr getReturnTypeImpl(const ColumnsWithTypeAndName & arguments) const override
         {
             FunctionArgumentDescriptors mandatory_args{
-                {"time", static_cast<FunctionArgumentDescriptor::TypeValidator>(&isString), nullptr, "String"},
-                {"format", static_cast<FunctionArgumentDescriptor::TypeValidator>(&isString), nullptr, "String"}
+                {"time", static_cast<FunctionArgumentDescriptor::TypeValidator>(&isString), nullptr, "String"}
             };
 
             FunctionArgumentDescriptors optional_args{
+                {"format", static_cast<FunctionArgumentDescriptor::TypeValidator>(&isString), nullptr, "String"},
                 {"timezone", static_cast<FunctionArgumentDescriptor::TypeValidator>(&isString), &isColumnConst, "const String"}
             };
 
@@ -2029,14 +2029,24 @@ namespace
 
         String getFormat(const ColumnsWithTypeAndName & arguments) const
         {
-            const auto * format_column = checkAndGetColumnConst<ColumnString>(arguments[1].column.get());
-            if (!format_column)
-                throw Exception(
-                    ErrorCodes::ILLEGAL_COLUMN,
-                    "Illegal column {} of second ('format') argument of function {}. Must be constant string.",
-                    arguments[1].column->getName(),
-                    getName());
-            return format_column->getValue<String>();
+            if (arguments.size() == 1)
+            {
+                if constexpr (parse_syntax == ParseSyntax::MySQL)
+                    return "%Y-%m-%d %H:%i:%s";
+                else
+                    return "yyyy-MM-dd HH:mm:ss";
+            }
+            else
+            {
+                const auto * col_format = checkAndGetColumnConst<ColumnString>(arguments[1].column.get());
+                if (!col_format)
+                    throw Exception(
+                        ErrorCodes::ILLEGAL_COLUMN,
+                        "Illegal column {} of second ('format') argument of function {}. Must be constant string.",
+                        arguments[1].column->getName(),
+                        getName());
+                return col_format->getValue<String>();
+            }
         }
 
         const DateLUTImpl & getTimeZone(const ColumnsWithTypeAndName & arguments) const
diff --git a/tests/queries/0_stateless/02668_parse_datetime.reference b/tests/queries/0_stateless/02668_parse_datetime.reference
index d21a51ce70c..b67ca2d8b76 100644
--- a/tests/queries/0_stateless/02668_parse_datetime.reference
+++ b/tests/queries/0_stateless/02668_parse_datetime.reference
@@ -239,7 +239,7 @@ select sTr_To_DaTe('10:04:11 03-07-2019', '%s:%i:%H %d-%m-%Y', 'UTC') = toDateTi
 select str_to_date('10:04:11 invalid 03-07-2019', '%s:%i:%H %d-%m-%Y', 'UTC') IS NULL;
 1
 -- Error handling
-select parseDateTime('12 AM'); -- { serverError NUMBER_OF_ARGUMENTS_DOESNT_MATCH }
+select parseDateTime(); -- { serverError NUMBER_OF_ARGUMENTS_DOESNT_MATCH }
 select parseDateTime('12 AM', '%h %p', 'UTC', 'a fourth argument'); -- { serverError NUMBER_OF_ARGUMENTS_DOESNT_MATCH }
 -- Fuzzer crash bug #53715
 select parseDateTime('', '', toString(number)) from numbers(13); -- { serverError ILLEGAL_COLUMN }
@@ -270,3 +270,7 @@ select parseDateTime('8 13, 2022, 7:58:32', '%c %e, %G, %k:%i:%s', 'UTC');
 2022-08-13 07:58:32
 select parseDateTime('08 13, 2022, 07:58:32', '%c %e, %G, %k:%i:%s', 'UTC');
 2022-08-13 07:58:32
+-- The format string argument is optional
+set session_timezone = 'UTC'; -- don't randomize the session timezone
+select parseDateTime('2021-01-04 23:12:34') = toDateTime('2021-01-04 23:12:34');
+1
diff --git a/tests/queries/0_stateless/02668_parse_datetime.sql b/tests/queries/0_stateless/02668_parse_datetime.sql
index 02ac0c5f35c..7b3aed60a4a 100644
--- a/tests/queries/0_stateless/02668_parse_datetime.sql
+++ b/tests/queries/0_stateless/02668_parse_datetime.sql
@@ -162,7 +162,7 @@ select sTr_To_DaTe('10:04:11 03-07-2019', '%s:%i:%H %d-%m-%Y', 'UTC') = toDateTi
 select str_to_date('10:04:11 invalid 03-07-2019', '%s:%i:%H %d-%m-%Y', 'UTC') IS NULL;
 
 -- Error handling
-select parseDateTime('12 AM'); -- { serverError NUMBER_OF_ARGUMENTS_DOESNT_MATCH }
+select parseDateTime(); -- { serverError NUMBER_OF_ARGUMENTS_DOESNT_MATCH }
 select parseDateTime('12 AM', '%h %p', 'UTC', 'a fourth argument'); -- { serverError NUMBER_OF_ARGUMENTS_DOESNT_MATCH }
 
 -- Fuzzer crash bug #53715
@@ -187,4 +187,9 @@ select parseDateTime('08 13, 2022, 07:58:32', '%m %e, %G, %k:%i:%s', 'UTC');
 select parseDateTime('8 13, 2022, 7:58:32', '%c %e, %G, %k:%i:%s', 'UTC');
 select parseDateTime('08 13, 2022, 07:58:32', '%c %e, %G, %k:%i:%s', 'UTC');
 
+-- The format string argument is optional
+set session_timezone = 'UTC'; -- don't randomize the session timezone
+select parseDateTime('2021-01-04 23:12:34') = toDateTime('2021-01-04 23:12:34');
+
+
 -- { echoOff }
diff --git a/tests/queries/0_stateless/02668_parse_datetime_in_joda_syntax.reference b/tests/queries/0_stateless/02668_parse_datetime_in_joda_syntax.reference
index 9fbf105dc41..6f560577ab5 100644
--- a/tests/queries/0_stateless/02668_parse_datetime_in_joda_syntax.reference
+++ b/tests/queries/0_stateless/02668_parse_datetime_in_joda_syntax.reference
@@ -354,5 +354,9 @@ select parseDateTimeInJodaSyntaxOrNull('2001 366 2000', 'yyyy D yyyy', 'UTC') =
 select parseDateTimeInJodaSyntaxOrNull('2001 invalid 366 2000', 'yyyy D yyyy', 'UTC') IS NULL;
 1
 -- Error handling
-select parseDateTimeInJodaSyntax('12 AM'); -- { serverError NUMBER_OF_ARGUMENTS_DOESNT_MATCH }
+select parseDateTimeInJodaSyntax(); -- { serverError NUMBER_OF_ARGUMENTS_DOESNT_MATCH }
 select parseDateTimeInJodaSyntax('12 AM', 'h a', 'UTC', 'a fourth argument'); -- { serverError NUMBER_OF_ARGUMENTS_DOESNT_MATCH }
+-- The format string argument is optional
+set session_timezone = 'UTC'; -- don't randomize the session timezone
+select parseDateTimeInJodaSyntax('2021-01-04 23:12:34') = toDateTime('2021-01-04 23:12:34');
+1
diff --git a/tests/queries/0_stateless/02668_parse_datetime_in_joda_syntax.sql b/tests/queries/0_stateless/02668_parse_datetime_in_joda_syntax.sql
index f5810d3d4c3..28d14607ba6 100644
--- a/tests/queries/0_stateless/02668_parse_datetime_in_joda_syntax.sql
+++ b/tests/queries/0_stateless/02668_parse_datetime_in_joda_syntax.sql
@@ -239,6 +239,11 @@ select parseDateTimeInJodaSyntaxOrNull('2001 366 2000', 'yyyy D yyyy', 'UTC') =
 select parseDateTimeInJodaSyntaxOrNull('2001 invalid 366 2000', 'yyyy D yyyy', 'UTC') IS NULL;
 
 -- Error handling
-select parseDateTimeInJodaSyntax('12 AM'); -- { serverError NUMBER_OF_ARGUMENTS_DOESNT_MATCH }
+select parseDateTimeInJodaSyntax(); -- { serverError NUMBER_OF_ARGUMENTS_DOESNT_MATCH }
 select parseDateTimeInJodaSyntax('12 AM', 'h a', 'UTC', 'a fourth argument'); -- { serverError NUMBER_OF_ARGUMENTS_DOESNT_MATCH }
+
+-- The format string argument is optional
+set session_timezone = 'UTC'; -- don't randomize the session timezone
+select parseDateTimeInJodaSyntax('2021-01-04 23:12:34') = toDateTime('2021-01-04 23:12:34');
+
 -- { echoOff }