diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/csv/CSVInferSchema.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/csv/CSVInferSchema.scala
index ae9f0571555e..03cc3cbdf790 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/csv/CSVInferSchema.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/csv/CSVInferSchema.scala
@@ -17,6 +17,8 @@
 
 package org.apache.spark.sql.catalyst.csv
 
+import java.util.Locale
+
 import scala.util.control.Exception.allCatch
 
 import org.apache.spark.rdd.RDD
@@ -32,7 +34,10 @@ class CSVInferSchema(val options: CSVOptions) extends Serializable {
     options.zoneId,
     options.locale)
 
-  private val decimalParser = {
+  private val decimalParser = if (options.locale == Locale.US) {
+    // Special handling the default locale for backward compatibility
+    s: String => new java.math.BigDecimal(s)
+  } else {
     ExprUtils.getDecimalParser(options.locale)
   }
 
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/csv/CSVInferSchemaSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/csv/CSVInferSchemaSuite.scala
index c2b525ad1a9f..24d909ed99b9 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/csv/CSVInferSchemaSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/csv/CSVInferSchemaSuite.scala
@@ -185,6 +185,8 @@ class CSVInferSchemaSuite extends SparkFunSuite with SQLHelper {
       assert(inferSchema.inferField(NullType, input) == expectedType)
     }
 
-    Seq("en-US", "ko-KR", "ru-RU", "de-DE").foreach(checkDecimalInfer(_, DecimalType(7, 0)))
+    // input like '1,0' is inferred as strings for backward compatibility.
+    Seq("en-US").foreach(checkDecimalInfer(_, StringType))
+    Seq("ko-KR", "ru-RU", "de-DE").foreach(checkDecimalInfer(_, DecimalType(7, 0)))
   }
 }
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/csv/CSVSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/csv/CSVSuite.scala
index 6584a30036fd..90deade2a51d 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/csv/CSVSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/csv/CSVSuite.scala
@@ -2052,4 +2052,11 @@ class CSVSuite extends QueryTest with SharedSQLContext with SQLTestUtils with Te
 
     checkAnswer(rows, expectedRows)
   }
+
+  test("SPARK-27512: Decimal type inference should not handle ',' for backward compatibility") {
+    assert(spark.read
+      .option("delimiter", "|")
+      .option("inferSchema", "true")
+      .csv(Seq("1,2").toDS).schema.head.dataType === StringType)
+  }
 }