diff --git a/docs/sql-migration-guide.md b/docs/sql-migration-guide.md
index c4e9d1f2a2569..e37e12f71ebb1 100644
--- a/docs/sql-migration-guide.md
+++ b/docs/sql-migration-guide.md
@@ -26,6 +26,7 @@ license: |
   
   - Since Spark 3.4, Number or Number(\*) from Teradata will be treated as Decimal(38,18). In Spark 3.3 or earlier, Number or Number(\*) from Teradata will be treated as Decimal(38, 0), in which case the fractional part will be removed.
   - Since Spark 3.4, v1 database, table, permanent view and function identifier will include 'spark_catalog' as the catalog name if database is defined, e.g. a table identifier will be: `spark_catalog.default.t`. To restore the legacy behavior, set `spark.sql.legacy.v1IdentifierNoCatalog` to `true`.
+  - Since Spark 3.4, the results of casting Decimal values as String type will not contain exponential notations. To restore the legacy behavior, which uses scientific notation if the adjusted exponent is less than -6, set `spark.sql.legacy.castDecimalToString.enabled` to `true`.
 
 ## Upgrading from Spark SQL 3.2 to 3.3
 
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala
index 45950607e0de9..5dd986e25e7b8 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Cast.scala
@@ -512,6 +512,7 @@ case class Cast(
     TimestampFormatter.getFractionFormatter(ZoneOffset.UTC)
 
   private val legacyCastToStr = SQLConf.get.getConf(SQLConf.LEGACY_COMPLEX_TYPES_TO_STRING)
+  private val legacyCastDecimalToStr = SQLConf.get.getConf(SQLConf.LEGACY_DECIMAL_TO_STRING)
   // The brackets that are used in casting structs and maps to strings
   private val (leftBracket, rightBracket) = if (legacyCastToStr) ("[", "]") else ("{", "}")
 
@@ -625,6 +626,8 @@ case class Cast(
     case DayTimeIntervalType(startField, endField) =>
       buildCast[Long](_, i => UTF8String.fromString(
         IntervalUtils.toDayTimeIntervalString(i, ANSI_STYLE, startField, endField)))
+    case _: DecimalType if !legacyCastDecimalToStr =>
+      buildCast[Decimal](_, d => UTF8String.fromString(d.toPlainString))
     case _ => buildCast[Any](_, o => UTF8String.fromString(o.toString))
   }
 
@@ -1475,6 +1478,8 @@ case class Cast(
             $evPrim = UTF8String.fromString($iu.toDayTimeIntervalString($c, $style,
               (byte)${i.startField}, (byte)${i.endField}));
           """
+      case _: DecimalType if !legacyCastDecimalToStr =>
+        (c, evPrim, _) => code"$evPrim = UTF8String.fromString($c.toPlainString());"
       case _ =>
         (c, evPrim, evNull) => code"$evPrim = UTF8String.fromString(String.valueOf($c));"
     }
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala
index 1b7857ead59fa..15bd5cb5e365c 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala
@@ -3697,6 +3697,17 @@ object SQLConf {
       .booleanConf
       .createWithDefault(false)
 
+  val LEGACY_DECIMAL_TO_STRING =
+    buildConf("spark.sql.legacy.castDecimalToString.enabled")
+      .internal()
+      .doc("When true, casting decimal values as string will use scientific notation if an " +
+        "exponent is needed, which is the same with the method java.math.BigDecimal.toString(). " +
+        "Otherwise, the casting result won't contain an exponent field, which is compliant to " +
+        "the ANSI SQL standard.")
+      .version("3.4.0")
+      .booleanConf
+      .createWithDefault(false)
+
   val LEGACY_PATH_OPTION_BEHAVIOR =
     buildConf("spark.sql.legacy.pathOptionBehavior.enabled")
       .internal()
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/types/Decimal.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/types/Decimal.scala
index 759a5dce967bf..f4f54d2f93bcd 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/types/Decimal.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/types/Decimal.scala
@@ -225,6 +225,8 @@ final class Decimal extends Ordered[Decimal] with Serializable {
 
   override def toString: String = toBigDecimal.toString()
 
+  def toPlainString: String = toBigDecimal.bigDecimal.toPlainString
+
   def toDebugString: String = {
     if (decimalVal.ne(null)) {
       s"Decimal(expanded, $decimalVal, $precision, $scale)"
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CastSuiteBase.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CastSuiteBase.scala
index 97cbc781829ef..da9a7dca9f158 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CastSuiteBase.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CastSuiteBase.scala
@@ -1305,4 +1305,12 @@ abstract class CastSuiteBase extends SparkFunSuite with ExpressionEvalHelper {
         Cast(child, DecimalType.USER_DEFAULT), it)
     }
   }
+
+  test("SPARK-39749: cast Decimal to string") {
+    val input = Literal.create(Decimal(0.000000123), DecimalType(9, 9))
+    checkEvaluation(cast(input, StringType), "0.000000123")
+    withSQLConf(SQLConf.LEGACY_DECIMAL_TO_STRING.key -> "true") {
+      checkEvaluation(cast(input, StringType), "1.23E-7")
+    }
+  }
 }