Reads[java.math.BigDecimal]

gistfile1.scala

import play.api.libs.json._
import play.api.libs.functional.syntax._
import scala.util.control.Exception._
import java.lang.NumberFormatException
import play.api.data.validation.ValidationError

implicit val bigDecReads = Reads[java.math.BigDecimal]( js => js match {
  case JsString(s) => 
    catching(classOf[NumberFormatException])
      .opt( JsSuccess(new java.math.BigDecimal(s)) )
      .getOrElse( JsError(ValidationError("validate.error.expected.numberformatexception")))
  case JsNumber(d) => JsSuccess(d.underlying)
  case _ => JsError(ValidationError("validate.error.expected.jsnumberorjsstring"))
} )

implicit val bigDecWrites = Writes[java.math.BigDecimal]( bd => JsString(bd.toString) )