diff --git a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
index 327f97a3cc9d..878b74a4e9fe 100644
--- a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
+++ b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
@@ -3665,11 +3665,14 @@ object Parsers {
           subExpr() match
             case rhs0 @ Ident(name) if placeholderParams.nonEmpty && name == placeholderParams.head.name
                 && !tpt.isEmpty && mods.is(Mutable) && lhs.forall(_.isInstanceOf[Ident]) =>
-              if sourceVersion.isAtLeast(future) then
-                deprecationWarning(
-                  em"""`= _` has been deprecated; use `= uninitialized` instead.
-                      |`uninitialized` can be imported with `scala.compiletime.uninitialized`.""",
-                  rhsOffset)
+              report.gradualErrorOrMigrationWarning(
+                em"""`= _` has been deprecated; use `= uninitialized` instead.
+                        |`uninitialized` can be imported with `scala.compiletime.uninitialized`.${rewriteNotice(`3.4-migration`)}""",
+                in.sourcePos(rhsOffset),
+                warnFrom = `3.4`,
+                errorFrom = future)
+              if sourceVersion.isMigrating && sourceVersion.isAtLeast(`3.4-migration`) then
+                patch(source, Span(rhsOffset, rhsOffset + 1), "scala.compiletime.uninitialized")
               placeholderParams = placeholderParams.tail
               atSpan(rhs0.span) { Ident(nme.WILDCARD) }
             case rhs0 => rhs0
diff --git a/compiler/test/dotty/tools/dotc/CompilationTests.scala b/compiler/test/dotty/tools/dotc/CompilationTests.scala
index 0491660219b2..21235ac048e2 100644
--- a/compiler/test/dotty/tools/dotc/CompilationTests.scala
+++ b/compiler/test/dotty/tools/dotc/CompilationTests.scala
@@ -60,6 +60,7 @@ class CompilationTests {
       compileFile("tests/rewrites/rewrites.scala", defaultOptions.and("-source", "3.0-migration").and("-rewrite", "-indent")),
       compileFile("tests/rewrites/rewrites3x.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
       compileFile("tests/rewrites/rewrites3x-fatal-warnings.scala", defaultOptions.and("-rewrite", "-source", "future-migration", "-Xfatal-warnings")),
+      compileFile("tests/rewrites/uninitialized-var.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
       compileFile("tests/rewrites/with-type-operator.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
       compileFile("tests/rewrites/private-this.scala", defaultOptions.and("-rewrite", "-source", "future-migration")),
       compileFile("tests/rewrites/filtering-fors.scala", defaultOptions.and("-rewrite", "-source", "3.2-migration")),
diff --git a/language-server/src/dotty/tools/languageserver/DottyLanguageServer.scala b/language-server/src/dotty/tools/languageserver/DottyLanguageServer.scala
index e7f9c332aeeb..c58b17e49559 100644
--- a/language-server/src/dotty/tools/languageserver/DottyLanguageServer.scala
+++ b/language-server/src/dotty/tools/languageserver/DottyLanguageServer.scala
@@ -15,6 +15,7 @@ import scala.collection._
 import scala.jdk.CollectionConverters._
 import scala.util.control.NonFatal
 import scala.io.Codec
+import scala.compiletime.uninitialized
 
 import dotc._
 import ast.{Trees, tpd, untpd}
@@ -54,14 +55,14 @@ class DottyLanguageServer extends LanguageServer
   import lsp4j.jsonrpc.messages.{Either => JEither}
   import lsp4j._
 
-  private var rootUri: String = _
+  private var rootUri: String = uninitialized
 
-  private var myClient: DottyClient = _
+  private var myClient: DottyClient = uninitialized
   def client: DottyClient = myClient
 
-  private var myDrivers: mutable.Map[ProjectConfig, InteractiveDriver] = _
+  private var myDrivers: mutable.Map[ProjectConfig, InteractiveDriver] = uninitialized
 
-  private var myDependentProjects: mutable.Map[ProjectConfig, mutable.Set[ProjectConfig]] = _
+  private var myDependentProjects: mutable.Map[ProjectConfig, mutable.Set[ProjectConfig]] = uninitialized
 
   def drivers: Map[ProjectConfig, InteractiveDriver] = thisServer.synchronized {
     if myDrivers == null then
diff --git a/language-server/test/dotty/tools/languageserver/util/PositionContext.scala b/language-server/test/dotty/tools/languageserver/util/PositionContext.scala
index 10629d900c92..1f73be867c06 100644
--- a/language-server/test/dotty/tools/languageserver/util/PositionContext.scala
+++ b/language-server/test/dotty/tools/languageserver/util/PositionContext.scala
@@ -3,9 +3,11 @@ package dotty.tools.languageserver.util
 import dotty.tools.languageserver.util.embedded.CodeMarker
 import dotty.tools.languageserver.util.server.TestFile
 
+import scala.compiletime.uninitialized
+
 class PositionContext(positionMap: Map[CodeMarker, (TestFile, Int, Int)]) {
-  private var lastKey: CodeMarker = _
-  private var lastValue: (TestFile, Int, Int) = _
+  private var lastKey: CodeMarker = uninitialized
+  private var lastValue: (TestFile, Int, Int) = uninitialized
   def positionOf(pos: CodeMarker): (TestFile, Int, Int) = {
     if (lastKey eq pos) lastValue
     else {
diff --git a/language-server/test/dotty/tools/languageserver/util/server/TestServer.scala b/language-server/test/dotty/tools/languageserver/util/server/TestServer.scala
index 749b92daba8c..473c14324860 100644
--- a/language-server/test/dotty/tools/languageserver/util/server/TestServer.scala
+++ b/language-server/test/dotty/tools/languageserver/util/server/TestServer.scala
@@ -7,6 +7,8 @@ import java.nio.file.{Files, Path}
 import java.nio.charset.StandardCharsets
 import java.util
 
+import scala.compiletime.uninitialized
+
 import dotty.tools.dotc.Main
 import dotty.tools.dotc.reporting.{Reporter, ThrowingReporter}
 import dotty.tools.io.Directory
@@ -17,7 +19,7 @@ import org.eclipse.lsp4j.{ DidOpenTextDocumentParams, InitializeParams, Initiali
 class TestServer(testFolder: Path, projects: List[Project]) {
 
   val server = new DottyLanguageServer
-  var client: TestClient = _
+  var client: TestClient = uninitialized
 
   init()
 
diff --git a/library/src/scala/util/control/NonLocalReturns.scala b/library/src/scala/util/control/NonLocalReturns.scala
index ad4dc05f36ac..c7e600b4c028 100644
--- a/library/src/scala/util/control/NonLocalReturns.scala
+++ b/library/src/scala/util/control/NonLocalReturns.scala
@@ -1,5 +1,7 @@
 package scala.util.control
 
+import scala.compiletime.uninitialized
+
 /** Library implementation of nonlocal return.
  *
  *  Usage:
@@ -21,7 +23,7 @@ package scala.util.control
 object NonLocalReturns {
   @deprecated("Use scala.util.boundary.Break instead", "3.3")
   class ReturnThrowable[T] extends ControlThrowable {
-    private var myResult: T = _
+    private var myResult: T = uninitialized
     def throwReturn(result: T): Nothing = {
       myResult = result
       throw this
diff --git a/presentation-compiler/src/main/dotty/tools/pc/MetalsDriver.scala b/presentation-compiler/src/main/dotty/tools/pc/MetalsDriver.scala
index 55504db7a11a..819c3f2fc9c9 100644
--- a/presentation-compiler/src/main/dotty/tools/pc/MetalsDriver.scala
+++ b/presentation-compiler/src/main/dotty/tools/pc/MetalsDriver.scala
@@ -7,6 +7,8 @@ import dotty.tools.dotc.interactive.InteractiveDriver
 import dotty.tools.dotc.reporting.Diagnostic
 import dotty.tools.dotc.util.SourceFile
 
+import scala.compiletime.uninitialized
+
 /**
  * MetalsDriver is a wrapper class that provides a compilation cache for InteractiveDriver.
  * MetalsDriver skips running compilation if
@@ -29,7 +31,7 @@ class MetalsDriver(
     override val settings: List[String]
 ) extends InteractiveDriver(settings):
 
-  @volatile private var lastCompiledURI: URI = _
+  @volatile private var lastCompiledURI: URI = uninitialized
 
   private def alreadyCompiled(uri: URI, content: Array[Char]): Boolean =
     compilationUnits.get(uri) match
diff --git a/project/Build.scala b/project/Build.scala
index 3ac19a0f7b35..ab4cfe19377b 100644
--- a/project/Build.scala
+++ b/project/Build.scala
@@ -1267,6 +1267,7 @@ object Build {
       ivyConfigurations += SourceDeps.hide,
       transitiveClassifiers := Seq("sources"),
       Compile / scalacOptions ++= Seq("-Yexplicit-nulls", "-Ysafe-init"),
+      Compile / scalacOptions ++= Seq("-source", "3.3"), // To avoid fatal migration warnings
       Compile / sourceGenerators += Def.task {
         val s = streams.value
         val cacheDir = s.cacheDirectory
diff --git a/sbt-test/compilerReporter/i14576/Test.scala b/sbt-test/compilerReporter/i14576/Test.scala
index d94a49145f81..8be852b846a0 100644
--- a/sbt-test/compilerReporter/i14576/Test.scala
+++ b/sbt-test/compilerReporter/i14576/Test.scala
@@ -10,8 +10,5 @@ object Test:
   def f(x: Text) = println(x.str)
   f("abc")
 
-  // private[this] and = _ are deprecated under -source:future
-  private[this] var x: AnyRef = _
-
   // under -source:future, `_` is deprecated for wildcard arguments of types: use `?` instead
   val xs: List[_] = Nil
diff --git a/sbt-test/compilerReporter/i14576/build.sbt b/sbt-test/compilerReporter/i14576/build.sbt
index f9f211b24977..cc0402a7ba5e 100644
--- a/sbt-test/compilerReporter/i14576/build.sbt
+++ b/sbt-test/compilerReporter/i14576/build.sbt
@@ -24,7 +24,7 @@ lazy val root = (project in file("."))
     },
     assertDeprecationSummary := {
       assert {
-        FakePrintWriter.messages.exists(_.contains("there were 2 deprecation warnings; re-run with -deprecation for details"))
+        FakePrintWriter.messages.exists(_.contains("there was 1 deprecation warning; re-run with -deprecation for details"))
       }
     },
     assertNoDeprecationSummary := {
diff --git a/tests/neg/i4812.scala b/tests/neg/i4812.scala
index 8d518107825c..c6f6dafc656c 100644
--- a/tests/neg/i4812.scala
+++ b/tests/neg/i4812.scala
@@ -1,6 +1,6 @@
 //> using options -Werror
 object Test:
-  var prev: Any = _
+  var prev: Any = scala.compiletime.uninitialized
 
   def test[T](x: T): T =
     class A(val elem: (T, Boolean))
@@ -55,7 +55,7 @@ object Test:
 
   def test6[T](x: T): T =
     class A { var b: B = null }
-    class B { var a: A = null; var elem: T = _ }
+    class B { var a: A = null; var elem: T = scala.compiletime.uninitialized }
     prev match
       case prev: A => // error: the type test for A cannot be checked at runtime
         prev.b.elem
@@ -88,7 +88,7 @@ object Test:
       case x: B => x
 
   sealed class A
-  var prevA: A = _
+  var prevA: A = scala.compiletime.uninitialized
   def test10: A =
     val methodCallId = System.nanoTime()
     class B(val id: Long) extends A
diff --git a/tests/neg/uninitialized-3.4.check b/tests/neg/uninitialized-3.4.check
new file mode 100644
index 000000000000..1c7b985072d0
--- /dev/null
+++ b/tests/neg/uninitialized-3.4.check
@@ -0,0 +1,6 @@
+-- Error: tests/neg/uninitialized-3.4.scala:7:15 -----------------------------------------------------------------------
+7 |  var a: Int = _ // error: migration warning
+  |               ^
+  |               `= _` has been deprecated; use `= uninitialized` instead.
+  |               `uninitialized` can be imported with `scala.compiletime.uninitialized`.
+  |               This construct can be rewritten automatically under -rewrite -source 3.4-migration.
diff --git a/tests/neg/uninitialized-3.4.scala b/tests/neg/uninitialized-3.4.scala
new file mode 100644
index 000000000000..174a95ae6c54
--- /dev/null
+++ b/tests/neg/uninitialized-3.4.scala
@@ -0,0 +1,8 @@
+//> using options -Werror
+
+import scala.language.`3.4`
+import scala.compiletime.uninitialized
+
+class Foo:
+  var a: Int = _ // error: migration warning
+  var b: Int = uninitialized
diff --git a/tests/neg/uninitialized-future-migration.scala b/tests/neg/uninitialized-future-migration.scala
new file mode 100644
index 000000000000..05f7c6b67f38
--- /dev/null
+++ b/tests/neg/uninitialized-future-migration.scala
@@ -0,0 +1,8 @@
+//> using options -Werror
+
+import scala.language.`future-migration`
+import scala.compiletime.uninitialized
+
+class Foo:
+  var a: Int = _ // error: migration warning
+  var b: Int = uninitialized
diff --git a/tests/neg/uninitialized-future.scala b/tests/neg/uninitialized-future.scala
new file mode 100644
index 000000000000..8882b70ed48b
--- /dev/null
+++ b/tests/neg/uninitialized-future.scala
@@ -0,0 +1,6 @@
+import scala.language.future
+import scala.compiletime.uninitialized
+
+class Foo:
+  var a: Int = _ // error
+  var b: Int = uninitialized
diff --git a/tests/patmat/i12805-fallout.scala b/tests/patmat/i12805-fallout.scala
index b598b36159ea..66d9c0d598fd 100644
--- a/tests/patmat/i12805-fallout.scala
+++ b/tests/patmat/i12805-fallout.scala
@@ -1,4 +1,5 @@
 import scala.annotation.unchecked.uncheckedVariance
+import scala.compiletime.uninitialized
 
 type Untyped = Null
 
@@ -7,7 +8,7 @@ class Type
 abstract class Tree[-T >: Untyped] {
   type ThisTree[T >: Untyped] <: Tree[T]
 
-  protected var myTpe: T @uncheckedVariance = _
+  protected var myTpe: T @uncheckedVariance = uninitialized
 
   def withType(tpe: Type): ThisTree[Type] = {
     val tree = this.asInstanceOf[ThisTree[Type]]
diff --git a/tests/pos/uninitialized-future-migration.scala b/tests/pos/uninitialized-future-migration.scala
new file mode 100644
index 000000000000..a1e606dc90fb
--- /dev/null
+++ b/tests/pos/uninitialized-future-migration.scala
@@ -0,0 +1,6 @@
+import scala.language.`future-migration`
+import scala.compiletime.uninitialized
+
+class Foo:
+  var a: Int = _ // warn
+  var b: Int = uninitialized
diff --git a/tests/rewrites/uninitialized-var.check b/tests/rewrites/uninitialized-var.check
new file mode 100644
index 000000000000..3809938512a7
--- /dev/null
+++ b/tests/rewrites/uninitialized-var.check
@@ -0,0 +1,2 @@
+class Foo:
+  var a: Int = scala.compiletime.uninitialized
diff --git a/tests/rewrites/uninitialized-var.scala b/tests/rewrites/uninitialized-var.scala
new file mode 100644
index 000000000000..910734b33350
--- /dev/null
+++ b/tests/rewrites/uninitialized-var.scala
@@ -0,0 +1,2 @@
+class Foo:
+  var a: Int = _