diff --git a/sdk/core/azure-core/CHANGELOG.md b/sdk/core/azure-core/CHANGELOG.md
index 829264ecf..63e8ae5e9 100644
--- a/sdk/core/azure-core/CHANGELOG.md
+++ b/sdk/core/azure-core/CHANGELOG.md
@@ -15,6 +15,9 @@
 
 - Fixed `Azure::DateTime::Parse()` validation if the result is going to exceed `9999-12-31T23:59:59.9999999` due to time zone, leap second, or fractional digits rounding up adjustments.
 - [[#3224]](https://github.com/Azure/azure-sdk-for-cpp/issues/3224) Fixed intermittent crash on macOS when logging is turned on.
+- The `Base64::Decode` API will throw a `std::runtime_error` exception if presented with invalid inputs.
+
+### Other Changes
 
 ## 1.3.1 (2021-11-05)
 
diff --git a/sdk/core/azure-core/src/base64.cpp b/sdk/core/azure-core/src/base64.cpp
index c2aa06d67..867e796bc 100644
--- a/sdk/core/azure-core/src/base64.cpp
+++ b/sdk/core/azure-core/src/base64.cpp
@@ -382,9 +382,15 @@ static void Base64WriteThreeLowOrderBytes(std::vector<uint8_t>::iterator destina
 std::vector<uint8_t> Base64Decode(const std::string& text)
 {
   auto inputSize = text.size();
-  if (inputSize < 4)
+  if (inputSize % 4 != 0)
   {
-    return std::vector<uint8_t>(0);
+    throw std::runtime_error("Unexpected end of Base64 encoded string.");
+  }
+
+  // An empty input should result in an empty output.
+  if (inputSize == 0)
+  {
+    return std::vector<uint8_t>();
   }
 
   size_t sourceIndex = 0;
@@ -406,6 +412,10 @@ std::vector<uint8_t> Base64Decode(const std::string& text)
   while (sourceIndex + 4 < inputSize)
   {
     int64_t result = Base64Decode(inputPtr + sourceIndex);
+    if (result < 0)
+    {
+      throw std::runtime_error("Unexpected character in Base64 encoded string");
+    }
     Base64WriteThreeLowOrderBytes(destinationPtr, result);
     destinationPtr += 3;
     sourceIndex += 4;
@@ -436,6 +446,11 @@ std::vector<uint8_t> Base64Decode(const std::string& text)
     i0 |= i3;
     i0 |= i2;
 
+    if (i0 < 0)
+    {
+      throw std::runtime_error("Unexpected character in Base64 encoded string");
+    }
+
     Base64WriteThreeLowOrderBytes(destinationPtr, i0);
     destinationPtr += 3;
   }
@@ -446,6 +461,10 @@ std::vector<uint8_t> Base64Decode(const std::string& text)
     i2 <<= 6;
 
     i0 |= i2;
+    if (i0 < 0)
+    {
+      throw std::runtime_error("Unexpected character in Base64 encoded string");
+    }
 
     destinationPtr[1] = static_cast<uint8_t>(i0 >> 8);
     destinationPtr[0] = static_cast<uint8_t>(i0 >> 16);
@@ -453,6 +472,11 @@ std::vector<uint8_t> Base64Decode(const std::string& text)
   }
   else
   {
+    if (i0 < 0)
+    {
+      throw std::runtime_error("Unexpected character in Base64 encoded string");
+    }
+
     destinationPtr[0] = static_cast<uint8_t>(i0 >> 16);
     destinationPtr += 1;
   }
diff --git a/sdk/core/azure-core/test/ut/base64_test.cpp b/sdk/core/azure-core/test/ut/base64_test.cpp
index efb860596..5272273c0 100644
--- a/sdk/core/azure-core/test/ut/base64_test.cpp
+++ b/sdk/core/azure-core/test/ut/base64_test.cpp
@@ -125,3 +125,49 @@ TEST(Base64, Roundtrip)
     EXPECT_EQ(Convert::Base64Decode(Convert::Base64Encode(data)), data);
   }
 }
+
+TEST(Base64, ValidDecode)
+{
+  // cspell::disable
+  EXPECT_NO_THROW(Convert::Base64Decode(Convert::Base64Encode(std::vector<uint8_t>{})));
+  EXPECT_NO_THROW(Convert::Base64Decode(""));
+  EXPECT_NO_THROW(Convert::Base64Decode("aa=="));
+  EXPECT_NO_THROW(Convert::Base64Decode("aaa="));
+  // cspell::enable
+}
+
+TEST(Base64, InvalidDecode)
+{
+  // cspell::disable
+  EXPECT_THROW(Convert::Base64Decode("a"), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("aa"), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("aaa"), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("a==="), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("===="), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("@#!%"), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("ABCD%GA="), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("ABCDE^A="), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("ABCDEF&="), std::runtime_error);
+
+  EXPECT_THROW(Convert::Base64Decode("ABD%GA=="), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("ABDE^A=="), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("ABDEF&=="), std::runtime_error);
+
+  EXPECT_THROW(Convert::Base64Decode("AD%GA==="), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("ADE^A==="), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("ADEF&==="), std::runtime_error);
+
+  EXPECT_THROW(Convert::Base64Decode("ABCD===="), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("ADEF====="), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("ADEF======"), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("QQ======"), std::runtime_error);
+
+  EXPECT_THROW(Convert::Base64Decode("AB===CD="), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("AB==CD=="), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("AB=CD==="), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("AB====CD"), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("AD=====EF"), std::runtime_error);
+  EXPECT_THROW(Convert::Base64Decode("AD======EF"), std::runtime_error);
+
+  // cspell::enable
+}
diff --git a/sdk/core/azure-core/test/ut/simplified_header_test.cpp b/sdk/core/azure-core/test/ut/simplified_header_test.cpp
index 610d3606b..e5db88367 100644
--- a/sdk/core/azure-core/test/ut/simplified_header_test.cpp
+++ b/sdk/core/azure-core/test/ut/simplified_header_test.cpp
@@ -33,7 +33,7 @@ TEST(SimplifiedHeader, core)
   EXPECT_NO_THROW(Azure::Core::Context c);
   EXPECT_NO_THROW(Azure::DateTime(2020, 11, 03, 15, 30, 44));
   EXPECT_NO_THROW(Azure::ETag e);
-  EXPECT_NO_THROW(Azure::Core::Convert::Base64Decode("foo"));
+  EXPECT_NO_THROW(Azure::Core::Convert::Base64Decode("foo="));
   EXPECT_NO_THROW(Azure::Core::Cryptography::Md5Hash m);
   EXPECT_NO_THROW(Azure::Core::Http::RawResponse r(
       1, 1, Azure::Core::Http::HttpStatusCode::Accepted, "phrase"));