diff --git a/stix2/test/v21/test_observed_data.py b/stix2/test/v21/test_observed_data.py
index 71bad46..abcbb7b 100644
--- a/stix2/test/v21/test_observed_data.py
+++ b/stix2/test/v21/test_observed_data.py
@@ -1300,6 +1300,7 @@ def test_software_example():
     s = stix2.v21.Software(
         name="Word",
         cpe="cpe:2.3:a:microsoft:word:2000:*:*:*:*:*:*:*",
+        swid="com.acme.rms-ce-v4-1-5-0",
         version="2002",
         vendor="Microsoft",
     )
diff --git a/stix2/v21/observables.py b/stix2/v21/observables.py
index e8c1925..622e933 100644
--- a/stix2/v21/observables.py
+++ b/stix2/v21/observables.py
@@ -760,6 +760,7 @@ class Software(_Observable):
         ('id', IDProperty(_type, spec_version='2.1')),
         ('name', StringProperty(required=True)),
         ('cpe', StringProperty()),
+        ('swid', StringProperty()),
         ('languages', ListProperty(StringProperty)),
         ('vendor', StringProperty()),
         ('version', StringProperty()),