diff --git a/.changeset/tall-symbols-enjoy.md b/.changeset/tall-symbols-enjoy.md
new file mode 100644
index 0000000..7a83fe2
--- /dev/null
+++ b/.changeset/tall-symbols-enjoy.md
@@ -0,0 +1,5 @@
+---
+"@obosbbl/validation": patch
+---
+
+add checksum validation of Swedish organization numbers. Previously we only checked the length of the number.
diff --git a/packages/validation/src/se.ts b/packages/validation/src/se.ts
index d3a3f14..d41cc2b 100644
--- a/packages/validation/src/se.ts
+++ b/packages/validation/src/se.ts
@@ -81,7 +81,7 @@ export function validateOrganizationNumber(
     value = stripFormatting(value);
   }
 
-  return /^\d{10}$/.test(value);
+  return /^\d{10}$/.test(value) && mod10(value);
 }
 
 type NationalIdentityNumberFormat = 'short' | 'long';