diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/csv/CSVInferSchema.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/csv/CSVInferSchema.scala
index c63aae9d8385..49a991f6ffa2 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/csv/CSVInferSchema.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/csv/CSVInferSchema.scala
@@ -85,7 +85,9 @@ private[csv] object CSVInferSchema {
         case NullType => tryParseInteger(field, options)
         case IntegerType => tryParseInteger(field, options)
         case LongType => tryParseLong(field, options)
-        case _: DecimalType => tryParseDecimal(field, options)
+        case _: DecimalType =>
+          // DecimalTypes have different precisions and scales, so we try to find the common type.
+          findTightestCommonType(typeSoFar, tryParseDecimal(field, options)).getOrElse(StringType)
         case DoubleType => tryParseDouble(field, options)
         case TimestampType => tryParseTimestamp(field, options)
         case BooleanType => tryParseBoolean(field, options)
diff --git a/sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/csv/CSVInferSchemaSuite.scala b/sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/csv/CSVInferSchemaSuite.scala
index 93f752d107ca..8620bb9f65b9 100644
--- a/sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/csv/CSVInferSchemaSuite.scala
+++ b/sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/csv/CSVInferSchemaSuite.scala
@@ -114,4 +114,21 @@ class CSVInferSchemaSuite extends SparkFunSuite {
     val options = new CSVOptions(Map("TiMeStampFormat" -> "yyyy-mm"))
     assert(CSVInferSchema.inferField(TimestampType, "2015-08", options) == TimestampType)
   }
+
+  test("SPARK-18877: `inferField` on DecimalType should find a common type with `typeSoFar`") {
+    val options = new CSVOptions(Map.empty[String, String])
+
+    // 9.03E+12 is Decimal(3, -10) and 1.19E+11 is Decimal(3, -9).
+    assert(CSVInferSchema.inferField(DecimalType(3, -10), "1.19E+11", options) ==
+      DecimalType(4, -9))
+
+    // BigDecimal("12345678901234567890.01234567890123456789") is precision 40 and scale 20.
+    val value = "12345678901234567890.01234567890123456789"
+    assert(CSVInferSchema.inferField(DecimalType(3, -10), value, options) == DoubleType)
+
+    // Seq(s"${Long.MaxValue}1", "2015-12-01 00:00:00") should be StringType
+    assert(CSVInferSchema.inferField(NullType, s"${Long.MaxValue}1", options) == DecimalType(20, 0))
+    assert(CSVInferSchema.inferField(DecimalType(20, 0), "2015-12-01 00:00:00", options)
+      == StringType)
+  }
 }