diff --git a/NEWS b/NEWS
index 2d28fecb..aa6ca5f1 100644
--- a/NEWS
+++ b/NEWS
@@ -1,4 +1,6 @@
 2.1.6.dev0
+ * #1300: Fix the "source_directories_must_exist" option to support source directories relative to a
+   "working_directory".
  * Add the NEWS changelog file to release tarball (#1298).
  * Enable reply by email on projects.torsion.org, so replies to notification emails get posted as
    comments on tickets.
diff --git a/borgmatic/borg/create.py b/borgmatic/borg/create.py
index 7d23e9d7..8e3e9820 100644
--- a/borgmatic/borg/create.py
+++ b/borgmatic/borg/create.py
@@ -180,7 +180,9 @@ def make_base_create_command(  # noqa: PLR0912
     open pattern file handle).
     '''
     if config.get('source_directories_must_exist', True):
-        borgmatic.borg.pattern.check_all_root_patterns_exist(patterns)
+        borgmatic.borg.pattern.check_all_root_patterns_exist(
+            patterns, borgmatic.config.paths.get_working_directory(config)
+        )
 
     patterns_file = borgmatic.borg.pattern.write_patterns_file(
         patterns,
diff --git a/borgmatic/borg/pattern.py b/borgmatic/borg/pattern.py
index aee6a780..261a1d63 100644
--- a/borgmatic/borg/pattern.py
+++ b/borgmatic/borg/pattern.py
@@ -88,16 +88,16 @@ def write_patterns_file(patterns, borgmatic_runtime_directory, patterns_file=Non
     return patterns_file
 
 
-def check_all_root_patterns_exist(patterns):
+def check_all_root_patterns_exist(patterns, working_directory):
     '''
-    Given a sequence of Pattern instances, check that all root pattern paths exist. If any don't,
-    raise an exception.
+    Given a sequence of Pattern instances and the current working directory, check that all root
+    pattern paths exist. If any don't, raise an exception.
     '''
     missing_paths = [
         pattern.path
         for pattern in patterns
         if pattern.type == Pattern_type.ROOT
-        if not os.path.exists(pattern.path)
+        if not os.path.exists(os.path.join(working_directory or '', pattern.path))
     ]
 
     if missing_paths:
diff --git a/tests/unit/borg/test_pattern.py b/tests/unit/borg/test_pattern.py
index 7ed5158c..fdd2939b 100644
--- a/tests/unit/borg/test_pattern.py
+++ b/tests/unit/borg/test_pattern.py
@@ -34,19 +34,36 @@ def test_write_patterns_file_appends_to_existing():
 
 
 def test_check_all_root_patterns_exist_with_existent_pattern_path_does_not_raise():
-    flexmock(module.os.path).should_receive('exists').and_return(True)
+    flexmock(module.os.path).should_receive('exists').with_args('foo').and_return(True)
 
-    module.check_all_root_patterns_exist([Pattern('foo')])
+    module.check_all_root_patterns_exist([Pattern('foo')], working_directory=None)
+
+
+def test_check_all_root_patterns_exist_with_existent_relative_pattern_path_and_working_directory_does_not_raise():
+    flexmock(module.os.path).should_receive('exists').with_args('foo').never()
+    flexmock(module.os.path).should_receive('exists').with_args('/working/foo').and_return(True)
+
+    module.check_all_root_patterns_exist([Pattern('foo')], working_directory='/working')
 
 
 def test_check_all_root_patterns_exist_with_non_root_pattern_skips_existence_check():
     flexmock(module.os.path).should_receive('exists').never()
 
-    module.check_all_root_patterns_exist([Pattern('foo', Pattern_type.INCLUDE)])
+    module.check_all_root_patterns_exist(
+        [Pattern('foo', Pattern_type.INCLUDE)], working_directory=None
+    )
 
 
 def test_check_all_root_patterns_exist_with_non_existent_pattern_path_raises():
     flexmock(module.os.path).should_receive('exists').and_return(False)
 
     with pytest.raises(ValueError):
-        module.check_all_root_patterns_exist([Pattern('foo')])
+        module.check_all_root_patterns_exist([Pattern('foo')], working_directory=None)
+
+
+def test_check_all_root_patterns_exist_with_non_existent_relative_pattern_path_and_working_directory_raises():
+    flexmock(module.os.path).should_receive('exists').with_args('foo').never()
+    flexmock(module.os.path).should_receive('exists').with_args('/working/foo').and_return(False)
+
+    with pytest.raises(ValueError):
+        module.check_all_root_patterns_exist([Pattern('foo')], working_directory='/working')